For instance ,which one is correct? And why?
Le numéro de série est marqué en dessous.
Le numéro de série est marqué dessous.
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One difference is that you can use a complement with "en dessous" ou "sous", whereas you can't with "dessous"; thus, you can say this;
These two words are prépositions.
When you use "dessous", which is not a preposition (so you can't use nouns after it) but an adverb, you must specify in the context the thing under which is what you are talking about;