as
Sobrecama (counterpane), Sobrecamas
Sobredicho (aforesaid), Sobredichos
Tapaboca (slap on the mouth), Tapabocas
Tiraboton (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--

Ferrocarril (railway), Ferrocarriles
Madreselva (honeysuckle), Madreselvas

In the following compound nouns formed of an adjective and a noun, the
adjective remains invariable in the plural, as--

Bajamar (low tide), Bajamares
Bajorelieve (bas-relief), Bajorelieves
Belladona (belladonna), Belladonas
Blancomanjar (blanc-mange), Blancomanjares
Plenamar (full tide), Plenamares
Salvoconducto (safe conduct), Salvoconductos
Salvaguardia (safeguard), Salvaguardias
Santa Barbara (powder magazine), Santa Barbaras

In compound nouns formed of two words, the first of which has suffered
the loss or change of a letter, this first component remains
invariable--

(N.B.--Many such words are found in Spanish.)

Artimana (trick), Artimanas
Barbicano (white-bearded man), Barbicanos
Barbilindo (beardless man), Barbilindos
Boquirrubio (rosy-lipped), Boquirrubios
Cojitranco (lame fellow--disparagingly), Cojitrancos
Cuellierguido (stiff-necked man), Cuellierguidos
Gallipavo (turkey), Gallipavos
Manirroto (spendthrift), Manirrotos
Marisabidilla (blue stocking), Marisabidillas
Ojinegro (black-eyed), Ojinegros
Ojizarco (blue-eyed), Ojizarcos
Patizambo (bandy-legged), Patizambos
Pechicolorado (robin redbreast), Pechicolorados
Pleamar (high tide), Pleamares
Tragicomedia (tragi-comedy), Tragicomedias
Viandante (wayfarer), Viandantes

The following make their plural as follows--

Bancarrota (bankruptcy), Bancarrotas
Coliflor (cauliflower), Coliflores
Dares y tomares (wrangling), used only in _pl._
Dimes y diretes (ifs, ands and buts), used only in _pl._
Don Diego de noche (four o'clocks--flower), Don Diegos de noche
Maritornes (ill-shaped woman), Maritornes
Parabien (compliment), Parabienes
Vaiven (vibration), Vaivenes

The following are examples of the large class of nouns (compound) formed
with a verb or adverb and a plural noun and which are used for both
numbers--

Azotacalles (idler)
Buscavidas (pryer)
Cortaplumas (penknife)