What size of finished square can you cut from a 2.5 inch strip?
Obviously, if your strip is only 2.5 inches wide, the largest square you can cut from it is 2.5 x 2.5 inches, which will be a 2 inch finished square, after accounting for a 1/4" seam allowance on all sides.
What if you need to make a finished 2 inch half square triangle from those 2.5 inch strips?
You probably learned that when making a pair of HSTs, the size of the two raw squares needs to be 7/8 inch (I go for a whole inch) larger than the finished size of the HST. It's a geometry thing. (I won't bore you with that, LOL.)
Hey! Wait a minute! That means the raw squares to make the HST need to start at 2 7/8 inches (3 inches for me!). Unless you have an amazing trick that I don't know about, I'm pretty sure you can't get a 2 7/8 inch or 3 inch square from a 2.5 inch strip! What to do?