The plural for nouns ending with -il are formed in one of two ways depending upon whether the -il is stressed or not.

1. If the -il is stressed, change the end to -is — o barril = the barrel (sing.) — os barris = the barrels (pl.)

2. If the il is not stressed, change the end to -eis — o fóssil = the fossil (sing.) — os fósseis = the fossils (pl.)