Are you ready for another little math session? We did knit hexagons yesterday and today it's quilt triangles.

The triangles in question are from my second block of the month quilt block. I'm making two block of the month quilts - the one my lqs is running this year (I'm halfway done with that block) and the one they ran last year (they had some extra kits available and I snatched up a complete set!). Last year's January block of the month was the Dutchman's Puzzle block, which is made up of eight triangle units called flying geese. Flying geese are really cool and very versatile. Sometimes they are lined up in rows like in this awesome quilt from Denyse Schmidt Quilts or sometimes they're used to make a star, like in my other January block of the month block. I actually decided to start with this Dutchman's Puzzle block first because the simpler flying geese arrangement seemed like it would be easier than that star within a star block. Uh uh.

But before we get to the problem, let's first look at how pretty my geese turned out. I was so proud of them. My very first triangles. So crisp and neat looking.

But then I noticed it. See the background fabric to the left of the point of the triangle. There's only supposed to be a 1/4" of the background there. That's because quilt pieces are generally sewn together with a 1/4" seam and are therefore cut larger than they're meant to be to account for that. But mine was too large. Or rather too small. One or the other. I wasn't sure. So I dashed off to the Button Box to see what I had done wrong. It turns out that there was actually an error in that pattern block! (It had been a year, so they'd forgotten that there was this error in the first block.) The instructions had you cut a square of the colored fabric, then cut the square in half diagonally to get each big triangle for the geese. But the square they had you cut was 1/8" too small! After knitting, it's hard to imagine that so small an error would make such a difference, but it does. If I sew two geese together to make one corner of the block, I'll get that bit of cream background between my triangles ruining the look of the piece and the piece would be smaller than it should be. And because the block is made up of four of these two-geese units, that error in the size is going to be multiplied four times over before the block is done. It's sort of like having your gauge be off - every stitch you're off will be multiplied over and over across the whole width or length of the piece you're knitting. It's not good.

Well the wonderful ladies at the store not only helped me find the error and found me more fabric to start my block again, they spent over a half an hour with me explaining all the relevant quilt math so I could make the block correctly and be able to check for this sort of error in the future. They were so awesome!

Here goes... if your quilt piece is a nice, lovely square and you're using a 1/4" seam, well then you only have to cut the piece 1/2" larger than desired so it will turn out to be the right size. Leave 1/4" on each side for the seam and you're perfect. Got it, square = 1/2". Triangles are, naturally, a little trickier. Because of the angles in the triangle, the 1/4" seam ends up leaving extra bits of triangle tips in the seam (see the little bunny ears in the pics above?), meaning you have to add more than 1/2" to the original square you divide.

If you're cutting the square in half diagonally to make what are called half-square triangles, then you've got to add, get this, 7/8" extra. What a crazy number, right? But it's true (see this link for a super mathy version of all this). And if you're making quarter square triangles (that's triangles cut from a square that is cut twice diagonally), you have to add 1 1/4". And check this out, if you want to cut long triangles from a rectangle, you need to make the rectangle 11/16" larger! Nutty!

So that is what they showed me. But in a very reassuring moment, they added, now go home and try it on some scrap fabric to be sure. "It's just math." I love that!