The BlocLoc half square triangle ruler is one of my favorite specialty rulers. It is for squaring up your half square triangles. I had sworn off of specialty rulers until I used one at a guild charity sewing day. I actually pulled it up on my phone while still there and ordered one. I have several specialty rulers that I have never used. My SIL bought a yarn shop several years ago. They had a quilt shop upstairs at one time. She found a bunch of rulers in the back room. She sold them to me for $25. Several $100 worth of rulers. I pull them out from time to time. Rarely use them.
I just wish some of the patterns had more info when you purchase them. Sometimes they say you need a certain ruler. Wish it would give info on whether you can make it without the ruler.