"The reason such a term should exist is for the likes of; Tommy Hutton with Tom Seaver, or Joe McEwing with Randy Johnson. Guys who usually are mediocre or terrible players, whom for whatever reason just play better against that pitcher (or hitter) or team" At least with a player-on-player match-up - as opposed to teams which are nothing more than constantly changing groups wearing the same laundry - a case could be made one player's skills match up very well or very poorly against an opponent and whatever trends develop might reasonably be expected to continue. But even there it's frequently over-stated. McEwing had two good games off Johnson - but since those were the first two (at least as a Met) the 6-for-8 with several XBHs streak he started with gave him the rep as a Unit-Killer. The problem was that after that he had about what you'd expect from a marginal player against a top pitcher -- on the order of 2 for his next 20 or something along those lines. Not that those minor inconvenient facts stopped people from believing that Joe had some sort of magic power against Long Tall Randy and that lineups should forever be re-arranged and better hitters sat down, and all because little Joe "had his number".