Why can't you use the word affiche for a poster?

Question

Accepted Answer

Hi Andrew, 
Une affiche  is more something used for advertising as the kind you find in French streets and also to advertise films.
Un poster  is something you use for decoration in your home.
Hope this helps!

Answer

Thank you.  So then the same piece of paper could be une affiche on the streets, and un poster in a teenager's bedroom.

Answer

Yes indeed, I don't think the word 'poster' existed when I was young as it comes directly from English but  all teenagers wanted to get their hands on 'affiches de cinéma' to decorate their bedrooms...