In Spanish, in constructions made up of two nouns that form one lexical unit in which the second noun modifies the first as if it were an , usually only the first one will be expressed as .

Examples:

horas punta (peak hours)
bombas lapa (naval mines)
faldas pantalón (culotte shorts)
ciudades dormitorio (commuter cities)
pisos piloto (model apartments)
coches cama (sleeper buses)
hombres rana (scuba divers)
niños prodigio (prodigy children)
noticias bomba (breaking news)
sofás cama (pullout sofa beds)

Nevertheless, there are cases in which the second noun can attain the status of full adjective and also be expressed as plural, as would normally be the case for adjectives.  This usually happens when the second noun can serve (with the same value) as an attribute of the first noun in .  Hence, it is acceptable to say:

Estados miembros (member states)
países satellites (satellite states)
empresas líderes (leading companies)
palabras claves (key words)
copias piratas (pirated copies)

To recap, both “palabras clave” or “copias pirata” as well as “palabras claves” or “copias piratas” are correct expressions.  In the first case, “clave” and “pirata” serve as nouns in and are left in the .  In the second case, they serve as full adjectives (with “clave” meaning fundamental and “pirata” meaning illegal or not authorized) and hence are expressed in plural form.

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