I taught at a retreat earlier this month, and some of my students struggled with their seam allowance. More specifically, they couldn’t wrap their head around the idea of measuring the patchwork instead of measuring the quarter-inch. Do you understand the difference?

I gave them this exercise. Use patches cut 1-1/2″ x 2-1/2″; five from light fabrics and five from something darker. Sew them together along the long edges, alternating fabrics. The resulting patchwork should be 10-1/2″ x 2-1/2″.

The idea is that you must measure the patchwork to determine if it’s the correct size. If it isn’t, then you have to adjust your seam allowance. The sample above measures 10-1/8″—a full 3/8″ too short. This in spite of the fact that I sewed my normal quarter-inch seam allowance.

You see, it doesn’t matter if I think my seam allowance is a perfect quarter-inch. If the patchwork isn’t coming out to the correct size, I have to adjust it.

Of the 18 people at this retreat, only three had units that measured 10-1/2″. That’s one out of six! Everyone else needed to adjust their seam allowance.

The reason it mattered so much is that we were creating small units for Petit Patchwork, shown above. The blocks are from 4″ to 8″. When your blocks are only 4″ or 6″ finished, there isn’t a lot of room to fudge.

Think of it this way. If you’re adding borders to a large quilt that’s 100″ x 100″, and your borders are half an inch too long, you can easily fudge in that 1/2″. That amount is half of one percent of the total. One-half of 1%.

But if you are making a 4″ block, and the block next to it is 4.5″ and you try to fudge it to fit, there’s no way. It won’t work because now that half an inch is 12.5% of the total!

It turns out that the smaller the pieces, the more important accuracy really is. Next time you’re struggling with accuracy, measure the patchwork. Adjust your seam allowance accordingly.