The X receiver is the designation traditionally given to the split end.
Naming of positions varies to some degree from coach to coach, but the split end is usually the X receiver. Note that split end is separate from wide receiver. All split ends are wide receivers, but all receivers are not split ends. Receivers do not have to be on the line of scrimmage, but split ends do.
After the X receiver is designated, the tight end is usually Y. He is Y if he is tight in the formation or splits wide himself. The first receiver off the line is usually Z. A fourth or fifth receiver is possible, and they usually get the designation of A or B.
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