27 = 27 Hertz — the reference frequency of the a‑chromatic scale.
1 = 1 Hertz — the reference frequency of the c‑chromatic scale.
It is one of the 12 scales of Natural Tuning.
Natural Tuning consists of the pitch reference frequency and of pure intervals, whose frequency ratios are the quotients of natural numbers.
All is based on the natural overtone series.
Just (pure) intonation is the only natural interval design.