z
Tracks your most used directories, based on 'frecency'
... with a rank lower than 1 are forgotten. Frecency is a portmanteau of 'recent' and 'frequency'. It is a weighted rank that depends on how often and how recently something occurred. As far as I know, Mozilla came up with the term. To z, a directory that has low ranking but has been accessed recently will quickly have higher rank than a directory accessed frequently a long time ago.