Dal manuale del giocatore pag. 64:
For example, Krusk the barbarian’s 4 ranks in Climb make his
Climb check results 4 points higher than they otherwise would be,
but even Gimble the bard, with no Climb ranks, can make a Climb
check because Climb can be used untrained. Gimble has a skill
modifier of –1 (+0 for his Strength, –1 for armor), but he can give it a
try. However, Gimble’s ranks in Use Magic Device let him do
something that he otherwise couldn’t do at all—namely, use a magic
item as if he had a particular spell on his class spell list that he
actually doesn’t have. Krusk, with no ranks in the skill, can’t make a
Use Magic Device check even at a penalty because Use Magic
Device can’t be used untrained.