It's not a "1", it's a "prime" and the root of that word is an ordinal (whence "...

posterboy on Oct 19, 2019 | parent | context | favorite | on: Music Theory for Musicians and Normal People

It's not a "1", it's a "prime" and the root of that word is an ordinal (whence "first") not a cardinal. There is no zero'th ordinal. There is just "none", which implies that A to A is simply not an interval.