Partial Seam Quilt Blocks: How to Sew Them, With a Size Chart

A partial seam is one seam you deliberately leave half sewn. You stitch it from the outer edge to about the middle of the center piece, stop, build the rest of the block around it, and only then go back and close it. The block ends up with no obvious starting point, which is exactly the look these blocks are after.

The Quilter's Bible files them under machine piecing and calls them right-angled seams or part-sewn seams. They are used when you join right-angled pieces of fabric, and to create woven effects in patchwork where it is hard to see where sewing begins or ends. Fons and Porter's Love of Quilting describes the same method as the way traditional blocks join patches and units around a central patch.

The technique is worth learning for one practical reason: a pattern that says partial seam is otherwise closed to you, and there is nothing in it harder than sewing a straight quarter inch seam and knowing when to stop.

What a partial seam is, and when a block needs one

Take a center square with four rectangles arranged around it, each rectangle overlapping the end of the one before. Try to sew it in rows and you cannot. There is no straight line that runs across the whole block, because every rectangle covers part of its neighbor. Ann of GourmetQuilter puts it well: the block looks innocent, it looks quick, and then you find there is no one long seam to get right the way through it.

The partial seam solves that. The first rectangle goes on only halfway, which leaves the square free along that edge while still holding the rectangle in place. Each of the other three then goes on as an ordinary full seam, and each one makes the next side long enough for the next piece. When the fourth is on, the block is the right size and the first seam can be closed.

The clue in a pattern is usually visual. Ann's rule of thumb is that anything that looks like it is rotating, anything with that pinwheel-style turn where every piece steps around the center, may well be built this way. If the pieces around the center are all the same size and the block still spins, a partial seam is what makes it possible.

It is not the only way to build a block that spins. A pinwheel is four half square triangles sewn into a four patch, and our pinwheel quilt block guide covers that one seam for seam. A partial seam belongs to the blocks where whole rectangles, not triangles, wrap the center.

The partial seam block size chart

The simplest partial seam block is a center square and four rectangles, and the whole thing runs on two numbers: the finished size of the block, and the finished width of the strips that go around it.

The two rules are these. The center finishes at the block size minus twice the strip width, because a strip sits on two opposite sides of it. Each rectangle finishes at the block size minus one strip width, because the fourth rectangle covers the end of the first. Add 1/2 inch to every piece for seam allowances and you have the cutting.

The 8 inch line is the one Ann demonstrates: a 4 1/2 inch center and four strips cut 2 1/2 inches wide by 6 1/2 inches long, for an 8 1/2 inch block that finishes at 8 inches in the quilt. Every other line in the chart is the same arithmetic with different numbers.

Two lines for a 12 inch block are there because the proportion is yours to choose. Narrow strips give a big center and a frame that reads as an outline. Wide strips give a small center and four strong bars that dominate the block. Neither is more correct, and the same pattern looks like two different blocks depending on which you pick.

Cutting from precuts and yardage

Strips are the easy part. A 2 1/2 inch strip, the standard jelly roll width, finishes at 2 inches, which is the 8 inch line of the chart. Cut it into 6 1/2 inch lengths and one strip gives you six rectangles, enough for one block with two to spare.

A 5 inch square trimmed down to 4 1/2 inches gives you the center for that same 8 inch block, which makes a charm pack and a jelly roll a matched pair for this block. Trimming a charm square is not a waste: it squares up an edge that has usually been through a lot of handling.

From yardage, cut the strips across the width of fabric and subcut them to length, and cut the centers from a contrasting piece. Our guide to cutting fabric accurately covers the ruler checks that keep a long strip from bowing, which matters here because the rectangle length is what sets the block size. If you want a center that holds its own against busy strips, solids do it without competing, and for strips in quantity there is fabric by the yard.

Method one: the square and four rectangles

This is the block every source starts with, because once you can sew it you can read any pattern that calls for a partial seam. Sherri McConnell of A Quilting Life walks through it in four minutes, and it is worth watching the piece go under the needle once before you try it.

The sequence, as The Quilter's Bible sets it out, is five steps.

One. Put the first rectangle right sides together with the square, aligning one side edge only. Sew that seam to the center of the square and stop. Press outward, away from the square.

Two. Pin the second rectangle right sides together with the center square and stitch the complete seam. Press outward as before. Repeat with the third rectangle, again sewing the complete seam.

Three. Position the final rectangle along the remaining edge of the square and sew the complete seam.

Four. Only the partially unsewn seam is left. Working from the back, pin the hanging portion in place against the rest of the block and sew the remainder of the seam.

Five. Press the completed block.

Two details decide whether this goes smoothly, and both come from Ann at GourmetQuilter, who has made a lot of these.

The first is where you stop. About halfway along the square is the target. With smaller pieces you might sew a little less, but do not stop too soon after you start, because at the end you have to get that seam back under the presser foot and you need something to hold on to. A seam that stops an inch in is a fiddle later.

The second is how much you press. Press only the first inch or so of that partial seam, out toward the strip and away from the square, and leave the rest of it alone. The last couple of inches are still going to be sewn, and pressing them flat now only means unpressing them later. Press the whole thing at the end.

Everything else is a normal quarter inch seam. Regular stitch length, regular everything, as Ann says. If your quarter inch is the part you are unsure about, a seam guide ruler gives you a fixed edge to sew against, and it matters more here than in a nine patch: the rectangle length sets the block size, and there is no second seam to absorb the difference.

One more thing about direction. Whichever way you go around the block, first to fourth, keep going the same way for every block in the quilt. The rectangles end up in the same rotation, and a single block turned the other way reads as a mistake once the top is together, even though every seam in it is perfect.

Method two: going around clockwise, with a backstitch

Fons and Porter's Love of Quilting teaches the same block with a few additions that make it steadier, particularly on blocks with points to protect.

Align the corner edges of the square and the first unit, right sides together, and stitch from the edge toward the center, backstitching at the halfway mark. That backstitch is the whole difference. It gives the partial seam a defined end, so it does not creep open while you handle the block, and it gives you somewhere precise to drop the needle when you come back to it.

Finger press the partial seam away from the square. Then work clockwise. Join the second unit by aligning the raw edges and flipping the block so the partially seamed unit is on top, stitching the entire length. Set the stitches with the iron, then press away from the square. Align the third unit, pin it, sew the full length, set the stitches and press away from the center square again. A spritz of starch here helps keep things crisp.

To join the fourth unit, fold the second unit out of the way, align the fourth, flip the block over and pin, then start at the center square and sew the entire length of that seam. Set the stitches and press away from the center.

To finish, fold the block along the partial seam so the raw edges align, drop the needle into the back-stitched area, and sew out to the edge. Set the stitches, open the block, and press the seams away from the center square.

Then check the block. Fons and Porter's last instruction is to make sure you still have a quarter inch seam allowance at each star point, and that check is the one most likely to catch a problem while it is still fixable. Points that have eaten their seam allowance will lose their tips when the blocks are joined.

Method three: the windmill block, pressed toward the center

Monica Poole of Pattern Poole TV builds a windmill block with a partial seam, and her version differs in two useful ways.

The first is pressing. She presses the partial seam toward the center square rather than away from it, and says so out loud as a rule she is breaking on purpose. The reason is bulk. In her block, the units around the center already carry the seams where triangles meet, and pressing everything outward would pile those up. Pressing in sends the allowances to the one place in the block that has no seams of its own.

The second is how she starts and ends. No reverse stitching at the start: she sews onto a leader and runs straight into the block, and stops a little past halfway. At the end, she sews the last seam starting from the center and working outward, so that if there is any extra fabric it gets eased toward the outer edge rather than pushed into the center square where four seams already meet.

Her finishing touch is a small one. With everything pressed toward the middle, the corners of the center square get thick, and she puts a dab of washable glue in each corner to hold the allowances flat, then presses from the right side to set it.

Pressing toward the center or away from it is not a matter of taste, then. It is decided by where the other seams already are. If the pieces around the center are plain rectangles, press away from the square, as The Quilter's Bible and Fons and Porter both do. If those pieces are pieced units carrying seams of their own, pressing in is what keeps the block flat. A wool pressing mat helps either way, because the heat comes back up through the block and the allowances settle where you put them, and a clapper is what flattens the thick corner without stretching it.

When a block needs two partial seams

Laura of SewVeryEasy uses partial seams at two scales in the same quilt, which is a good demonstration of how far the idea goes.

Her block starts with a small center square surrounded by four units, joined exactly as above: one partial seam to start, three full seams going around, then back to close the first. That gives a finished center section. Then the same section is treated as a center in its own right, with four much larger units going around it, and the whole rotation happens again: one partial seam, three full seams, then the first seam closed.

Two partial seams for the entire block, one at each scale. Everything else is a straight seam, sewn on and off the edge.

Her practical notes are worth repeating. Press to the dark side. Finger press the partial seam rather than taking it to the iron, and do not press the open part at all until the end. When you sew the partial seam at the larger scale, stop somewhere before the end rather than counting on halfway, because what you need is room to work, not a precise measurement.

This is also why patterns built on partial seams often look far harder than they are. Laura's whole quilt is four large blocks joined with sashing, and the pieces are big. It looks complicated because nothing lines up in rows, and it sews quickly for exactly the same reason.

Pressing a block that is not finished yet

Pressing is where partial seam blocks are usually lost, because for most of the block you are pressing something that is still incomplete.

The partial seam itself gets the lightest treatment. Ann presses about the first inch of it, away from the square, and no more. Laura does not take it to the iron at all and finger presses instead. Fons and Porter finger press it too, and only set the stitches with the iron once a full seam has gone on. All three are protecting the same thing: the tail of that seam has to stay loose and unpressed so it can be folded back and sewn at the end.

The three full seams get normal treatment. Set the stitches, then press the allowance in whichever direction the block calls for, and press each one before you add the next piece. Pressing as you go is faster here than pressing at the end, because each new piece lands on a flat edge instead of a folded one.

Then there is the last seam. Once it is closed, press the partial seam along its whole length in the same direction as its neighbor, and press the block from the right side to check that nothing has folded over on itself at the center.

Starch or a pressing spray, used lightly, is the thing that keeps all this crisp, and Fons and Porter reach for it at exactly the point where the block is most likely to distort. The allowance directions matter more in this block than in most, because every seam converges on the same small square.

Where partial seams go wrong

Four things account for most of the trouble, and all four are avoidable.

Stopping too soon. The most common one. A partial seam that runs only an inch or so leaves you nothing to grip when you fold the block back at the end, and the seam you are trying to finish sits right at the edge of the presser foot. Sew to about the middle of the center square and the last step becomes ordinary sewing.

Pressing the whole partial seam early. A pressed fold across the unsewn tail is a crease you then have to sew through, and it pulls the finished seam out of line. Press the first inch and leave the rest.

Forgetting the last seam. It sounds unlikely until you have a pile of blocks on the table and no idea which ones are closed. The fix is to finish each block before starting the next, or to keep the unfinished ones facing the same way in a single stack.

Bulk at the center. Four seams meet at one small square. If all four allowances are pressed toward it, the middle becomes a lump that a walking foot will find later. Decide the direction before you start, follow it for all four seams, and use a clapper on that center once the block is closed.

There is a fifth, quieter problem: a block that comes out slightly small. Because each rectangle sets the length of the next side, a seam that runs a thread wide accumulates around the block rather than canceling out. If the block finishes under size, the quarter inch is where to look first, not the pressing.

Chain piecing a stack of these blocks

Partial seam blocks are slower than a nine patch, and they are slower in a specific place. Ann's advice is the practical answer: you cannot chain piece the partial seam itself, and you cannot chain piece the last seam either, because both start part way along.

What you can chain piece is everything in the middle. So the efficient order for a batch of blocks is this. Sew all the partial seams first, one at a time, and press the first inch of each. Then chain piece the second rectangle onto every block, press, chain piece the third, press, chain piece the fourth, press. Then close all the partial seams, one at a time, and give each block its final press.

That turns a block with two awkward moments into a production line with two short manual stages at either end of it.

A stiletto earns its place in this batch. Feeding the folded block for that last seam is the one moment where fingers get close to the needle, and the stiletto holds the fold down and keeps the layers from shifting as they go under the foot.

Blocks and settings that use a partial seam

The technique shows up at three different scales, and recognizing which one you are looking at tells you how to plan the sewing.

At block scale, it is the square and four rectangles, the windmill, and anything else where units turn around a central patch. Fons and Porter describe exactly that family: traditional blocks that join patches and units around a central patch. Ann's own version alternates her pieced blocks with plain squares in the quilt, which gives the eye somewhere to rest when the blocks are made from strong colors.

At unit scale, the pieces going around the center are not plain rectangles but pieced units. Monica's windmill puts a quarter square triangle unit with a small half square triangle attached on each side of the center, and the partial seam is what lets those four units rotate. If you want to build the units first, our guides to half square triangles and to the friendship star block cover the pieces that most often end up in this position.

At quilt scale, whole blocks are set around a center with partial seams, which is how Laura's four large blocks go together and how a medallion quilt grows out from its center. Pat Sloan, talking about a quilt whose setting is built this way, makes a point worth borrowing: people sometimes say inset seam when they mean partial seam, and the words get swapped in conversation. The constructions are different, and it is worth knowing which one a pattern means before you cut.

If you are choosing a first project for the technique, a nine block sampler of the plain version is hard to beat, and our beginner quilt patterns page has the simpler blocks to pair it with.

Turning partial seam blocks into a quilt

These blocks are square, they are easy to make in any size from the chart, and they behave like any other block once they are closed. That makes the quilt part simple.

Set them edge to edge and the frames form a woven grid across the quilt, which is the effect The Quilter's Bible mentions: it becomes hard to see where any seam begins or ends. Alternate them with plain squares, the way Ann does, and each block reads as its own object with space around it. Use two colorways of the same block and alternate those instead, and the quilt takes on a checkerboard rhythm without a single extra seam.

Sashing is the third option, and it is what Laura uses at the largest scale. Her sashing is deliberately close in value to the background so it blends in, which keeps the eye on the blocks rather than on the grid between them.

Whichever setting you choose, the finishing is ordinary. Our guide to borders, backing and binding covers the measuring and the math for any quilt top, and a quilt size chart is the faster way to decide how many blocks you need before you start cutting.

The free partial seam block sheet

Everything above fits on one page, and that page is free: the size chart for seven block sizes, the five step diagram, and the three things that decide how the block turns out. Partial seam quilt block pattern sheet, PDF. Print it, keep it by the machine, and you will not have to remember which seam was the open one.

Partial seam questions we are asked most

What is a partial seam in quilting?

It is a seam sewn only part of the way along, left open, and finished at the end of the block. The Quilter's Bible also calls it a right-angled seam or a part-sewn seam, and files it with machine piecing rather than with anything advanced.

The purpose is structural. In a block where each piece overlaps the end of the piece before it, there is no seam that runs across the whole block. Leaving one seam open for a while creates a straight edge that was not there, and closing it at the end locks the rotation in place.

How far along do you sew a partial seam?

About halfway along the center square is the working answer, and every source gives roughly the same figure. Ann of GourmetQuilter says to come about halfway, a couple of inches on a 4 1/2 inch center. Monica Poole sews halfway or a little more. Fons and Porter backstitch at the halfway mark.

The warning is about the other end of the range. Do not stop too soon after starting. You have to be able to get that seam back under the presser foot at the end, and there needs to be enough sewn length to hold everything steady while you do.

Do you backstitch a partial seam?

Both answers are in use. Fons and Porter backstitch at the halfway mark, which gives the seam a firm stopping point and a precise place to drop the needle later. Monica Poole does not, and starts on a leader instead so the stitches begin cleanly at the edge of the block.

The practical difference is how much handling the block gets before you close that seam. If you are making one block at a time and finishing it straight away, either works. If the blocks sit in a pile while you chain piece the other seams, the backstitch is worth the two seconds.

Which way do you press the seams?

Away from the center square when the surrounding pieces are plain, which is what The Quilter's Bible and Fons and Porter both do. Toward the center square when the surrounding pieces are pieced units carrying their own seams, which is what Monica Poole does in the windmill block to keep the bulk out of the corners.

Decide before you sew the first seam and keep all four the same. Mixed directions are what make a center that will not lie flat.

Can I press the partial seam straight away?

Only the first inch of it, out toward the strip. The rest of that seam is still going to be sewn, and a crease across the unsewn tail has to be sewn through later. Finger pressing it is enough, and both Laura at SewVeryEasy and Fons and Porter do exactly that.

Can you chain piece blocks with partial seams?

The middle seams, yes. The partial seam and the closing seam, no, because both begin part way along a seam line rather than at a raw edge. Sew all the partial seams first, chain piece the three full seams in three passes, then close the partial seams one by one.

My block came out smaller than the chart. What went wrong?

Look at the quarter inch first. In this block the length of each rectangle sets the length of the next side, so a seam that runs a thread wide does not cancel out, it accumulates as you go around the center.

The second place to look is the pressing. An allowance that has not been pressed fully flat takes up width, four times over, and the block reads small even though every piece was cut correctly. Press each seam before adding the next piece.

Is a partial seam the same as a Y seam or an inset seam?

No, although the words get swapped. Pat Sloan says as much while working on a quilt whose setting uses partial seams: she sometimes says inset seam when she means partial seam, and she is not the only one.

The difference is in what the seam has to do. A partial seam is a straight seam between two straight edges, and the only unusual thing about it is that you sew it in two sittings. A Y seam or set-in seam joins three pieces at a point and has to stop and pivot at a marked spot. If a pattern says partial seam, nothing in it needs pivoting.

Can I use precuts for a partial seam block?

Yes, and the sizes line up neatly. A 2 1/2 inch strip cut into 6 1/2 inch lengths gives the four rectangles for an 8 inch finished block, and a 5 inch square trimmed to 4 1/2 inches gives the center. Charm squares, jelly roll strips and a rotary cutter are the whole shopping list for that version.

Does the partial seam show in the finished quilt?

No. Once it is closed and pressed it is an ordinary seam, indistinguishable from the other three. That is the point of the construction: The Quilter's Bible describes these blocks as creating woven effects where it is hard to see where sewing begins or ends.

From the back it is equally invisible, provided the last stretch was pressed in the same direction as the rest of that seam. If you can pick out the partial seam on the front of a finished block, look at the pressing rather than the stitching.

Can I quilt over the center without trouble?

Yes, as long as the bulk was dealt with when the block was pressed. Four seam allowances converge at the corners of the center square, and a walking foot climbing that lump is where stitches change length.

The two fixes are the ones already in the block: choose a single pressing direction for all four seams, and flatten the corners with a clapper before the top goes together. A little washable glue in each corner, the way Monica Poole uses it, keeps them flat until they are quilted.

What size should my first one be?

An 8 inch block with 2 1/2 inch strips is the friendliest starting point, and it is the one Ann demonstrates. The center is large enough that halfway is a comfortable distance to sew, the strips are a standard precut width, and four blocks make a finished 16 inch panel that tells you quickly whether you enjoy the technique.

What to have ready

Nothing on this list is exotic, and most of it is already on the table.

An accurate quarter inch. Whatever gives you one, whether that is a marked plate, a guide taped to the bed, or a seam guide ruler. This block punishes a wandering seam more than most.

A square ruler for trimming. The block should measure the chart size before it is joined to anything, and a square ruler tells you that in one placement. Our quilting rulers collection has the larger sizes for 10 and 12 inch blocks.

A sharp blade. Rectangle length is what sets the block size, so a blade that drags and pushes the fabric costs you accuracy at the cutting stage. Fresh rotary blades are the cheapest accuracy you can buy.

A pressing setup you trust. A wool pressing mat for the four seams, and a clapper for the center once the block is closed.

A stiletto. For the folded last seam, a stiletto keeps the layers together at the needle. The rest of the small tools for this kind of work live in notions.

Fabric with contrast at the center. The block only reads as a rotation if the center separates from the frame, which is why a solid center under printed strips works so reliably. Solids for the centers, fabric by the yard for the strips.

Where to start

Cut one center at 4 1/2 inches and four strips at 2 1/2 by 6 1/2 inches, in two fabrics that contrast. Sew the first strip on halfway and stop. Add the other three, pressing each as it goes on. Fold the block back, close the first seam, and press.

The block should measure 8 1/2 inches. If it does, you have the technique, and every pattern that says partial seam is now open to you. If it measures a little under, the chart and the quarter inch will tell you why long before the next block goes wrong.