Are you one of those who prefers on top or underneath? Don't get me wrong! I'm talking about sleeping in a bunk bed.

And while we're at it: would you sleep 'encima suyo' or 'encima de él'?

The answer depends on whether the word is a noun or an adverb. Adverbs—encima, debajo, delante, detrás, enfrente—do not admit possessives. And the reason is simple: possessives modify nouns, not adverbs.

I say al lado mío because there is a mi lado. But not debajo suyo, because there is no such thing as su debajo.

Imagine you're always late and you have to sit in the back rows. You can do it alrededor de alguien, a su alrededor, or alrededor suyo: all three are correct. You can also sit detrás de esa persona. But you cannot sit en su detrás. That's why detrás suyo is not correct.

This use of possessives with adverbs is a more common error than you might think, it's one of those that comes from colloquial speech. But with a little attention, you'll distinguish it effortlessly.