Proto-Basque: *suɫɦo
Meaning: 1 hole 2 cave 3 make holes, dig holes 4 hollow, gully
Bizkaian: zulo, zilo, zillo 1, zulotu, zilotu 3, (Aulestia) sulo 1
Gipuzkoan: zulo 1
High Navarrese: zulo 1, ziloka 2, xillotu 3, (Goizueta) zolo 1
Low Navarrese: zulho, zilho 1, ziloka 2, zilotu, xillotu 3, (Isturitze) ziloka 4
Salazarese: zilo, zillo 1
Lapurdian: zulho, zilho, xilho 1, ziloka 2, zilotu, zilhatu 3
Baztanese: ziloka 4
Zuberoan: zilo, xilo 1, ziloka 2, zila(tü) 3
Roncalese: xillo 1, xillotu 3
Comments: The diminutive/expressive forms xilo, xillo, etc. have become usual in ZBR and RNC. As noted by Mitxelena, medieval place names have only zulho or zulo, thus the oldest form is postulated to have been *suɫɦo. This is a component of compounds such as (GIP) ipurt-zulo 'anus', eztar-zulo 'pharynx', (ZBR) südür-xílo 'nostril', etc. Cf. Lezgi sulu-r 'throat'.
