13.23.1 Summary

1) Sous (under) is a preposition, dessous (down) is an adverb. They have different functions in phrase.

Examples adverb vs. preposition
Elle va dessous.
  She goes down. (not: Elle va sous.)
Mets le dessous.
  Put it down there. / Put it under it. (not: Mets le sous.)
Va dessous.
  Go down. (not: Va sous.)

2) Further there are variations of dessous, called pronominal adverbs, meaning that they can be used without related objects/verbs.

Il y avait un tas de papier là-dessous.
  There was a lot of paper on it.
Ci-dessous on voit la photo.
  Down here, one can see a photo.

3) Is the object not vertically under another object, but more below/beneath it, dessous is used together with another preposition to take the function of a preposition.

En dessous de ma fenêtre, il y a un arbre.
Au dessous de ma fenêtre, il y a un arbre.
  Below my window, there is a tree.

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