Most quilts are ‘straight set’ – that is the blocks are set square to the edges of the quilt; sometimes there are sashing strips between the blocks, sometimes not. Sashing strips can make a quilt larger and they can also act as coping strips when pieced individually around each block as they enable you to the trim your blocks to all the exact same size.
But sometimes we need to set the blocks on point. They are much larger when set this way, so you don’t need as many to make a big quilt and they are often set with alternate plain blocks which allow you to show off your quilting. Again they may or may not have sashing strips between them.
Having decided to set your blocks on point though, you are left with the problem of the edges. Do you make extra complete blocks to fill in those gaps and then trim everything straight? Or do you fill in with plain triangles? Should your triangles be the same fabric as the border, or the sashing, or something else entirely? But, perhaps more crucially, what size do you cut those filler triangles and what sort of triangles are they?
If you just want to turn blocks on point individually then you use half-square triangles as you want the shorter edges to be the straight grain. And you can use this method to give the illusion of having set them on-point.
But if you have stitched the blocks into diagonal rows for an ‘on-point’ setting then you need the side triangles (that fill in the gaps and give the quilt a straight edge) to have the longer side on the straight grain – QSTs. The corner triangles are HSTs.
Now you know what sort of triangles you need to cut, how do you work out what size to cut them?
The magic number you need is 1.414 (or if you are doing sums in your head and just want a rough answer to be going on with you can use 1.5). Take the finished size of your blocks – say it is 12 inches. You need a QST where the short side (the one on the bias) is 12 inches. This side though is the one cut diagonally across the square, not the side of the square. But remembering 1.414 saves you having to play around with those dimly remembered squares on hypotenuses. Side triangles are QSTs – you get four from a square – so multiply your 12 inches by 1.414 (= 16.968) and add the 1¼ inch seam allowance for QSTs to get 18.218. This is roughly 18¼ (18.25) inches – we’re not going to go as tiny as sixteenths. So you will need to cut your side triangles from an 18¼ inch square, and remember you will get four from each square. So for the London Roads quilt shown above you would need to cut just two squares to give you the eight side triangles.
For your corner triangles you need HSTs – you get two of these from each square. This time divide the finished size of the block by 1.414 and add the HST seam allowance of ⅞. So, 12 ÷ 1.414 + 0.875 = 8.48 + 0.875 = 9.36, which you can round up to 9⅜ (9.375) inches or even 9½ .
It’s usually better to cut your setting triangles slightly larger than you might need – you can always trim them down, it’s easier than adding ‘bandages’ when you’ve cut things too small.
And what if we had done these sums in our heads using 1.5 instead of 1.414? QST would be 12 x 1.5 = 18 + 1.25 = 19¼ so it would be an inch too big whereas HST would be 12 ÷1.5 = 8 + 0.875 = 8⅞ which is a bit too small. But they at least give you an idea of how big or small a piece of fabric you might need.
I have done a table of the measurements you might need for setting your quilts on-point and you can download it here.