2-limit
A 2-limit interval is either an integer whose only prime factor is 2, or the reciprocal of such an integer. Naturally, since 2/1 is the octave, this limits us to unisons, octaves and stacks of octaves. The 2-limit is fundamental to any sort of EDO, and in fact, 1edo is basically the 2-limit in full, only with additional primes tempered to either the octave or the unison.
Due to octave equivalence as driven by pitch-chroma matching, the 2-limit also collapses very easily, with this collapse being generally implemented in lattices. This collapse is helpful to understanding the pitches involved in other prime axes within the space of a single octave.