For the first part of wide receiver Michael Crabtree’s career, he played split end. Meaning that typically, Crabtree was lined up on the opposite side from the tight end. In most West Coast offenses, the split end or “X” was clearly the number two receiver. They usually lined up on the quarterback’s left side and were often relegated to a third or even fourth option.
The flanker was usually the star of the West Coast offense. Jerry Rice was a flanker or “Z” while John Taylor made his living as the more obscure "X".
But now, coaches have moved Crabtree from the X to the Z (the Y is the tight end usually). With variable schemes teams play, the X and Z are nearly interchangeable. Nevertheless, the switch to Z for Crabtree is significant and might be one of the reasons Crabtree has seen 24 passes thrown his way in the last two games.
When asked about the switch and whether Crabtree will be more featured in his new role, quarterback Alex Smith said, “Well, especially I mean, didn’t Jerry Rice play Z? Greatest receiver of all time.”