Why is my block not square?
 

This is a BOM club block from my LQS that I am audtioning in scrap fabric. As a novice, I was very careful in my cutting, seam allowances, and pressing. But the strips hoizontally are 1/4" short.but the vertical length (joined strips) is the correct 13". Why would the block be perfect one direction, but not the other? Please help me understand what I did wrong before I cut into my good fabric. Thanks.

OOPS! wrong picture
 

Let's try again. Don't know how that happened, and couldn't figure out how to edit out that one. Sorry!

You are asking the wrong person - I recently did a small quilt that was supposed to measure 26" square finished - it measured 25 one way and 25.5 the other and it looks just fine - not sure where my problem was, but pretty sure it was all the hourglass blocks because I know the center block was the perfect size because it was applique and I did it on an oversize block and cut it down. I know it is all in the 1/4" seam allowances.

I can't see anything that would have thrown it off..it all fits together so perfectly, and the points look great...

Sometimes it is the amount taken up by last long seams. Due to the bulk of fabric at intersecting seams, the seam allowance needs to be adjusted. I cannot tell from the picture if the short width corresponds to the long seams that join the rows.

When I did the first 18 inch Civil War Tribute block, I was half an inch off. Told to use a thinner thread and a scant 1/4 inch. The second time, it came out fine. It is always a challenge to me.

sweetana3 has it right. The long seams need to be at least a thread width narrower.

I took a wall hanging class one time and we sewed long strips together for the background. I sew them all the same direction, top to bottom. It actually curved by the time I was done. Teacher told me I should have turned it and alternated sewing. Maybe that is why?

Beautiful block, but don't have any answers.

I'd like to start by saying ... that is a very good block! And bravo to you for doing a test block with scraps to work the kinks out!

Are you measuring your 1/4" seams AFTER ironing?

Are you using fabrics of different weights (ie - looks like you have muslin weight in there and the dark green might be a heavy weight fabric?). When I'm sewing small pieces (1/4" finished) or a block with a lot of pieces, I have to be very careful to either not use fabrics of different weights (ie I can't mix batiks and asians), OR adjust my seam allowance with every combination of fabric. The difference is that the heavier fabrics take up more space to fold over the seam than a batik or a muslin. The smaller the piece - or the more pieces you have (such as your block) can throw the whole block off.

You can test your 1/4" by pulling 3 of the fabrics you plan to use, cut each piece 1 or 2 inches (whichever is more consistent with the size you will be using) and sew them together. If you used 1" pieces your finished size will be 2.5" AFTER you iron it. If not, adjust.

And of course sewing over seam allowances needs to be compensated for as well.

Every time you have a seam you have an "opportunity" for the finished product to be off a smidge. The more seams you have, the larger the discrepancy in the end product. Each of these little smidges will multiple - naked to the eye until you look at the end result.

It's best to make sure that the parts within the block as perfect as possible - matching all seams and nice points. It's not at all uncommon that the outer edges of blocks are not perfect as a result (for all the above reasons) especially on a complicated block! I think the only blocks I can get to finish perfect on all edges is a square in a square! As long as you can trim the outer edges of the block and still have enough seam allowance to connect it to the next block (or sashing) you are OK.

Good luck!

Wow, I have been quilting a while, by no means an expert, but had no idea to compensate for the thicker seam allowances and some of the other great suggestions! My blocks are normally "off" a little and maybe now I can fix that. Thanks!!

BTW, your block is really nice.