When we change compound nouns into their plural forms, there are some general guidelines that we can follow for each category. That said, it is always wise to consult a dictionary when using these nouns; there are many exceptions to the general rules outlined here!
NB: There are some notable exceptions to this general rule, namely la mezzaluna --> le mezzelune and la mezzanotte --> le mezzenotti.
Noun + Adjective
In these cases, both words comprising the compound noun are pluralized.| la terracotta ... le terrecotte la cassaforte ... le casseforti il caposaldo ... i capisaldi la cartastraccia ... le cartestracce |
Adjective + Noun
In these cases, the noun is pluralized.| il francobollo ... i francobolli il mezzogiorno ... i mezzogiorni l'altoparlante ... gli altoparlanti il gentiluomo ... i gentiluomini |
NB: There are some notable exceptions to this general rule, namely la mezzaluna --> le mezzelune and la mezzanotte --> le mezzenotti.