They were either masculine or neuter, although neuters were rare. Masculine and feminine nouns declined alike, while neuters had different forms in the nominative/accusative/vocative. Other nouns in this declension were feminine; there were no neuters. Masculines and neuters belong to this declension. The only exception are the few neuters that are inflected as s-stems. The "z"-stems descended from PIE acrostatic neuters in -"os"/"es"-. It is possible that many reconstructible "a"-stem neuters originally belonged to this class, however. There are few reconstructible neuters; those that can be reconstructed were irregular. The neuters of this class continued the Indo-European proterokinetic neuters in "*-men-". The Spanish neuters "lo" and "ello" have no plural forms. Neuters, however, have merged the two types in favor of the short-stem endings.