There are some rules in Major League Baseball that have "evolved" over time. One such example is the checked swing rule. Announcers talk about the bat "breaking the plane of the plate" to judge whether or not the batter checked his swing. The official MLB rules, however, say that a strike should be called if the hitter made an attempt to hit the pitch. It makes no mention of an imaginary "plane" that the bat must not break, yet it is common that this imaginary delimiter is used.
What other "traditions" like this exist in Major League Baseball that are not in the official rules, but have existed for so long that they are treated as such?