What's the difference between “dessous” and “en dessous”?

Question

For instance ,which one is correct? And why?

Le numéro de série est marqué en dessous.

Le numéro de série est marqué dessous.

Answer 1

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;

• le numéro de série est marqué en dessous de/sous la date de fabrication.

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;

• La date de fabrication se trouve au dos du couvercle; le numéro de série se trouve dessous. The number is under the date.