Index number

An index number is a number used internally by games to identify certain things. In the Pokémon games, there are index numbers for many things, most which are not seen other than by the programmers.

Most specifically, there are index numbers for locations, items, moves, abilities, and Pokémon, all of which will (usually) change between generations. There are a few for each, however, that remain unused, or are filled with "junk" data, such as the famed glitch Pokémon Missingno., or useless items, such as Teru-sama.

Strangely, only Generation I had a Pokémon's index number and National Dex number not line up, at least for the most part, unlike subsequent generations. There is, however, some discrepancy between the index and National numbers, after Generation II. For example, in Generation III, Chimecho is National Pokédex number 358, but its index number is 411. Alternate forms of a Pokémon, most notably Unown in Generation III and Wormadam in Generation IV, also will have differing index numbers to accomodate their differences.