When is a Runner "Between" Bases?

[MD Longhorn]

The basepaths form a square. At each vertex, there's a 90 degree area bounded by the lines from the square, extended outward - anyone in that area is "on" the base wrt passing. If you're not within that 90 degree area, then your position on the basepaths can be determined relative to which line you're closest to by drawing a line from you directly to (and perpendicular to) that baseline.

Reyes was in that area - to pass him, the succeeding runner would have had to go beyond 3rd base.

IMHO, of course. There are others that simply draw a line between the center of the square (approximately the rubber) to the player, treating the basepaths as a circle. Even in that case, Reyes was not COMPLETELY passed by the succeeding runner ... and to be passed, you must be wholely and completely passed.