The elements which concur to the formation of compound nouns are—
A B
Verbs Adjectives
Prepositions Nouns
Adverbs Participles
Conjunctions Pronouns
Prefixes
GENERAL RULE—
The elements in A are invariable except:
Un bullebulle (busybody), Unos bullebulles
Un quehacer (occupation), Unos quehaceres
Pasapasa (sleight of hand), Pasapasas (sleight of hand tricks)
The elements in B take the mark of the plural:
Alzacuello (minister's collar) Alzacuellos
Alzapaño (curtain hook), Alzapañios
Andaniño (child's cart), Andaniñios
Antepasado (ancestor), Antepasados
Ave María (a prayer), Ave Marías
Cualquiera (whatever), Cualesquiera
Entremetido (intruder, busybody), Entremetidos
Gentilhombre (man of gentle birth), Gentileshombres
Guardafuego (fire-guard), Guardafuegos
Hijodalgo (squire), Hijosdalgo
Pasamano (handrail, lace-edgings), Pasamanos
Pasatiempo (pastime), Pasatiempos
Picaparte (latch or latchkey), Picapartes
Pisaverde (beau), Pisaverdes
Portaestandarte (standard bearer), Portaestandartes
Portafusil (musket-sling), Portafusiles
Puercoespín (porcupine), Puercosespines
Quienquiera (whoever), Quienesquiera
Quitasol (parasol), Quitasoles
Ricahembra (woman of gentle birth), Ricashembras
Sobrecama (counterpane), Sobrecamas
Sobredicho (aforesaid), Sobredichos
Tapaboca (slap on the mouth), Tapabocas
Tirabotón (button-hook), Tirabotones
N.B.—When the second of the two words commences with r this letter should be doubled—
Ropa (clothes), guardarropa (wardrobe), guardarropas (pl.)
In compound nouns (very rare) formed of two nouns of which the first stands as an adjective to the second, the first noun remains invariable, the second only taking the mark of the plural—