Cutting Corner and Setting Triangles for Diagonal Sets

I love a diagonal set for a quilt, I think it's far more interesting than a straight set. And that's what this quilt has, is a diagonal set. That simply means that the blocks are put on there diagonal and the rows are sewn together diagonally. At the beginning and end of each row, however, you need to have some sort of a square or some sort of a unit to finish that row. And that happens to be a triangle. So with a diagonal set we call these triangles here, setting triangles and the ones that go in the four corners, corner triangles. What's unusual about them though is that we need pay very, very careful attention to how we cut those triangles. When we are cutting the setting triangle, we want to make sure that we don't cut a square and then cut it in half diagonally because we'll end up with bias out here on this edge. Instead we want to cut a square and cut it in half diagonally twice. So that this is the straight edge and the bias is in here and gets sewn into the body of the quilt. And the straight edge, or the grain edge, is on the outside edge of the quilt. The opposite is true with our corner squares. The corner triangle needs to have the bias here, across here, so that this is the crosswise and widthwise grain. So I'm gonna show you how to do the math for this. All you need to know in the beginning is the finished size of your block. These blocks are 10 inches finished but we're gonna be working with a 12 inch finished block here for our math. And then we have a couple of magic numbers that we use. Because we're going to be cutting our setting triangles differently from our corner triangles, we're going to do the match a little bit differently too. The corner triangle again, we want our bias to be right here, and our straight of the grain, true grain, on the outside corner. So we're gonna cut a square and cut it in half diagonally. So we need to know how big to make that square. What we're going to do is we're going to take the finished size of our block and in this case we're working with 12 inch finished block. And we're going to divide it by 1.414. Then we're going to add a seam allowance of 7/8 inches. And the reason we're adding 7/8 inches is because we're bisecting a square in half diagonally, thus making a half square triangle. And to that, we always add 7/8 of an inch. So I've got the 12 inch block, I'm dividing it by 1.414, which yields 8.49. I'm gonna add 0.875, which is what 7/8 is, to that 8.49 and it yields 9.365, which is nine and three eights. Now if I don't want to deal with that 3/8, I can cut it nine and a half, I can always round up. My piece will be a little bit too big but I can trim it down, I don't have to worry about dealing with that three eights inch, instead of a half inch which is easier to cut. I'll need two of those squares, one will go in each corner. So I want to show you what happens. So I've got one of those squares cut already, and when I pull on this edge, I get a little bit of stretch. When I pull on this edge, I get a little bit of stretch. but when I pull across here, I get tons of stretch. So when I cut this in half diagonally once. Watch this distortion. So if that's left on the outside edge of my quilt, I can be in trouble. I don't want that kind of stretch on the outside edge of my quilt. I want that kind of stretch to be sewn directly into one of my rows first. So that the edges are more true and not stretchy. So that's how we do our corner triangles. Then our setting triangles, which are these triangles here on the sides, we're gonna use that same magic number 1.414. And the same net number, which is the finished size of our block, 12 inches. Only this time, we're gonna multiply instead of divide. So 12 multiplied by 1.414 is 16. And now, because we're gonna be cutting this in half diagonally twice, which is essentially the same as doing a quarter square triangle, we're gonna be adding 1.25 to it. So we take 16, 1.25 is 17.25 or 17 and a quarter. I have a big square here that's 17 and a quarter. If I pul these edges, we have a little bit of stretch. But if I pull through here, we have tons of stretch. So we're gonna cut this in half diagonally twice. And because I don't have a big ruler, I'm going to fold that in half. And if I put my, an inch mark on that fold, then I know it's true there. And right through that corner, then I'm good. Like so. Except I've got that nick in my blade, which keeps giving me trouble. And then I'm gonna cut this in half again. And again, if I had my long ruler, I could do it without having to measure along the top here. I could just go from point to point. But I can also do it this way, just as accurately. And now I've got a triangle that is very different than my first triangle, not just in size but how it functions. Because now, I've got a true edge here. So look at that stretch, versus that stretch. You don't want this on our outside edge. So now we've got our bias stretch in here and here and our true grain out here. And that's how you measure and cut for your setting triangles and your corner triangles in a diagonal set.