Cowgill's Law states that between a Proto-Indo-European labial (p, bh, kw, w, etc.) and a resonant (l, m, n, etc.), *o becomes y (upsilon) in Greek.
*nokwts 'night' > nyx
*bholyom 'leaf' > phyllon
*kwetwores 'four' > (Homeric) pisyres, (Lesbian) pesyres
*H3nomn= > (Aeolic, Doric) onyma; cf. anōnymos.
Words like polys 'many' and polis 'city' are anomalies under this law., for expected pylys, pylis.
Named after Warren Cowgill.