also: position letters · receiver letters · skill letters
The letters offenses use to name skill players. X, Y and Z are fairly consistent; H and F vary considerably by program.
The common convention is useful, but it is not a standard. X is often the split end, Z the flanker and Y the tight end. In some two-tight-end systems, Y is the attached blocker while F or U is the tight end who moves around the formation. Another offense may use F for a fullback. H may be a slot receiver, wing or H-back.
The letters survive flipping the formation. Calling a route for Z works to either side; calling it for the right receiver does not. A new playbook still has to define every letter before the formation names mean anything.
position letterssplit end, flanker and slot, personnel grouping