Originally Posted by Prism99

There is a specific technique you can use to apply sashings accurately every time. All you need to do is mark your long sashings with where the seams should go.

If your rows consist of 12-inch blocks with 1-inch sashings, for example, you would mark your long sashing strip at 12 inches, at the next 1 inch, at the next 12 inches, at the next 1 inch, etc. You can use pencil on the wrong side of the sashing strip to make these marks, which should go from one cut side of the strip to the other. When you are ready to attach your sashing to a row of blocks, pin each intersection first. You will find that in some places you will need to ease in a block and in other places you will need to ease in the sashing.

Most importantly, when you attach the next row of blocks, you again pin each intersection. That way the blocks in the 2nd row will be aligned perfectly with the blocks in the 1st row.

When you are pinning, if you encounter a large discrepancy that can't be eased to fit, that is the time to fix the origin of the problem. Looking at the picture you provided, I think you would have found that the 2nd-to-
end block in the top row was too large to be easily matched up to the block below (which might have been a bit too small). At that point you could probably have prevented the problem by unsewing the short sashing and re-sewing it so that top block was a little smaller.

Cornerstones are actually a more elaborate solution to the problem. In effect, they create the seam intersections needed for matching rows. Marking the sashing accomplishes the same thing when cornerstones are not used.