if you are making HSTs, both pieces should be the same size. I'm not sure, but it sounds like you are making snowball blocks, with 'flippy corners'. if so, when you sew, you don't sew exactly on the line, you sew right next to the line, a bit towards the outside corner. The other thing is if the added triangle is slightly smaller, like an eighth or less, don't cut all 3 excess layers. Just cut the 2 inner layers and leave the original corner on the larger piece and use this edge when joining your blocks. The loose edges of the slightly smaller triangle will be caught between the larger piece and the adjoining patch making it secure. If you think of it, you can use a bit shorter stitch length for even more insurance.

The trick for you would be not to cut the larger piece. Use that as your guide because that is the final size it needs to be. You might have to sew a smiggen over from the line. Having said that, if you always seem to have problems, I would suggest purchasing the Triangulation 3.0 CD. It cost about $30 but has paper piecing patterns for every size possible for half square triangle, flying geese, etc. (you will need a printer). Every time you make one of these units, they would be perfect. Good luck.

I prefer to make 8 at a time using two large squares. They are a little oversized so I use the Quilt In A Day triangle square up ruler to cut one cut for squaring up. So easy and quick.
For 2 1/2 in triangles use 6 inch squares.
I spent twenty dollars for Triangulations and don't even use it.

I was just watching a Deb Tucker video on using her flying geese ruler, she sets the top square just a thread or two inside the bottom square, rectangle - whatever. As I read these comments, I suppose it has the same effect as sewing just outside the drawn line. I have not yet tried her method yet, and of course she is all about making them a little bigger and then trimming to perfect. Now I can't wait to get home and experiment.

I am talking about when doing snowballs or flying geese. The quilt I am currently working on is Bonnie Hunter's Smokey Mountain Stars, and you have a 4-1/2" square that you sew a 2" square onto a corner.

So i guess we are not really talking about half square triangles? For the flip and sew for snow ball type blocks I draw my line and sew just to the right of the line or towards the corner. If that makes sense

I also do this. What you may not be including when you draw your line is the 1/4 seam allowance. If you are making HST. the squares should be the same.

I consider myself an accomplished quilter, but have to say that I just love the Tucker Trimmer (Deb Tucker) for making HST, Hour glass, Shaded 4 patch. They are oversized and what is trimmed off is marginal. I also have her Rapid Fire Hunter's Star, V-Block and Wing Clipper for Flying Geese. My points are always perfect!

I recently read, or saw a video(who knows where) that when you draw that line, you should consider it the the pressing line, not the stitch line, so sew just off to the right of it, so when you press, it come out to the correct size.

I think what she is saying is that after sewing her seam, there is a small amount of fabric that has been pushed past the end of her stitching line. I have noticed that when using a less expensive fabric specially on the top of pieces being sewn, that the pressure feet push the top fabric piece, in essence stretching it past your bottom fabric. This has happened to me more times than I wish to count, until I figured out what was happening. Now I lay my two pieces of fabric together, spray starch & press them TOGETHER, then pin at both beginning & ending of that seam I'm going to sew. When I use higher quality fabric I do not have this problem. It happens more so if you are sewing cut triangles because your sewing the bias, and it really stretches.