Thursday, October 16, 2008

Clearing up the quilting math

Now, this is just here for anyone who actually was interested in the math for cutting quilt blocks. If you aren't interested, go read the funnies or something.
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Here is the block I will use to explain the math in regards to cutting different quilt shapes for a block. I would never make this block, but you can see each element in it easily.
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Since this block is a form of nine patch--based on nine squares--I would make it in a finished size that would be easily divisible by three. Either 12, 9, or 6 inches. Now that would be the finished size--what it would measure once it is sewn into a quilt. Before it is sewn into the quilt, it is half an inch larger, allowing for a quarter inch seam allowance all around.
The red square would be cut 4½ inches for a 12 inch block If the block were going to be a simple nine patch--nine plain squares, all the blocks would be the same.
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Those small squares--the brown and cream ones-- are ¼ of size of the red one. Finished size in the 12 inch block would be two inches, so they would be cut 2½ inches. (I'll just go on with the 12 inch size.)
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When I said yesterday that you have to take the fact that a rectangle would have one less seam, it was a bit confusing. (Probably not as confusing as this is.) That blue rectangle--finished--is twice the size of the red square, and the tan one is twice the size of the brown square. But when you cut them, you do not cut them 4½ by 9. You add half an inch to the 4 by 8 finished size and cut 4½ by 8½ (2½ by 4½ for the small ones). The purple rectangle is not 3 x 4½ but 3 x 4 plus the half inch.
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The orange triangles are half square triangles. You see--the are a square cut in half. To get those right, add 7/8 inches to the size of the finished square. So, our orange triangles are cut from squares that are 4-7/8 inches.
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The green triangles are quarter square triangles. You see--they are a square cut into quarters. Although they are the same shape as the half square triangles, you cut them differently so that you do not have a stretchy bias edge on the square. To cut quarter square triangles you start with a square that is 1¼ inches larger than the finished size of your final square--in this case, 5¼ inches.

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Don't get me started on odd shapes. I can demonstrate them, but I can't explain them.

7 comments:

Daryl said...

Math overload but fascinating none the less .. and I think your quilts and quilting knowledge rocks!

:-Daryl

PERBS said...

Thanks! I needed to know how to make triangles. I am saving this to my computer--not just the link but the whole post so I can print it and refer to it.

Now all I want to know is how to make the log cabin pattern. I mean the extra measurements for it. PLEASE.

I appreciated your "ugly" square for the purpose of instruction. Blessings!

Dragonstar said...

It all sounds so very complicated! I'm very glad of the explanation nonetheless.

Mary said...

That is a nice summary of the math of quilting! I always have trouble remembering what size to start with for quarter square triangles.

Aileni said...

You really should have a dedicated teaching site. Your compendious knowledge needs collating into an available reference blog for novices.

Or a book, of course. You could get paid for that.

CrazyCath said...

I had absolutely NO idea how much work would go into this. I take my hat off to you.

Gabriele said...

Kathy, the block might not be pretty lol, but it's great to explain the math that goes into the different shapes within the block. I wrote up a progressive sampler pattern based on those basic shapes and it's neat to see when the light bulb moments start popping up with the students.
I'm sure a lot more kids would like math if it's broken down in the way you did. :o)