Originally Posted by JARblue

Each pipe looks like it's letter. If you need a Y-pipe, you won't be able to use an X or H-pipe.

A Y-pipe connects to the both cats (your HFCs) on one end and a mid-pipe with a single outlet on the other end.

X and H-pipes have two outlets on each end in a true dual exhaust design. The crossover is to balance the exhaust pressures.

This image is a decent example of X and H-pipes. The Y-pipe shown is different than the Z because the Z transitions back to a dual exhaust in the mid-pipe (unless you have aftermarket exhaust with true dual exhaust design).