Elke from Germany asked me how I get the sq in sq units so precise. There are lots of ways of making these, including sewing strips around a square, and sewing oversize triangles to the centre square, both of which require trimming down to size. The strip method wastes fabric and the outer edges are on the bias, a recipe for disaster with these little pieces.

My method relies on two things you should be aiming for anyway, accurate cutting and a scant quarter inch seam allowance. If you can master those two, you can make perfect square in squares.

These instructions make a 3 1/2" unit. Start with two 2 3/8" squares for the background, and a 2 5/8" for the centre.
Cut the squares in half diagonally- I layer them and cut both at once- and then nub the ends at the two inch mark. This measurement is arbitrary, the main reason to get rid of these corners is to aid in centreing the triangle on the square underneath. The two inch mark works for me, but you could try others, just don't go below it or you'll be cutting too much off.

Lay the triangle over the centre square and use the trimmed ends to centre the piece. Make sure both ends are showing the same amount of the underneath fabric in those little spaces, and it will be in the right place.
When you sew, don't automatically use the nubbed corners as the starting point for the seam. They may well be, depending on how you nipped those bits off, but I usually have to sew a few threads closer to the edge.


I make sure I'm sewing my usual 1/4" seam, using the edge of my foot, or whatever guide you choose. If you sew from one nipped off point to the other the block may end up slightly smaller than you want.

Sew another triangle to the opposite side and then finger press them open carefully before you iron them. If you leave a little fold at this stage it will throw the final measurement off as well.

Centre the next triangle the same way, making sure there is the same amount of fabric showing at each end.

In this photo you can see a bit better that the same thing applies, those little corners don't have to match up with the edges underneath. Centre the triangle and sew the seam, that's all you have to worry about.

No trimming required and once you've got the hang of this method you can churn them out in a production line, and have a drawer full of them, like me.
Fiddle around and see if you can make this work for you. Everyone usually makes a few adjustments for their own sewing style, but this has worked for me for hundreds, if not thousands of these units. Good luck!