Regular MyISAM tables make use of B-tree indexes.

String indexes are space-compressed, which reduces the size of VARCHARs that don't use the full length, or a string that has trailing spaces. String indexes also make use of prefix-compression, where strings with identical prefixes are compressed.

Numeric indexes can also be prefix-compressed compressed if the PACK_KEYS=1 option is used. Regardless, the high byte is always stored first, which allows a reduced index size.

In the worst case, with no strings being space-compressed, the total index storage space will be (index_length+4)/0.67 per index.