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'Páginas web', 'palabras clave' and other plurals where only the first noun changes

Spanish often builds a new term by placing two nouns side by side: página web, palabra clave, año luz, hombre lobo. When you need the plural, only the first noun normally changes. You say páginas web, palabras clave, años luz and hombres lobo, and the second noun stays exactly as it was.

English speakers trip over this for two reasons. English puts the plural on the last word (web pages, light-years, werewolves), so the instinct is to add an -s at the end: páginas webs, años luces. And because the two Spanish words are separate, it feels natural to pluralize both. Neither instinct is right here.

Word list
cámara webfA2 webcam cámaras web página webfA2 website páginas web sitio webmA2 website sitios web carril bicimB1 bike path carriles bici (Spain) oso pandamB1 panda osos panda año luzmB2 light year años luz comida basurafB2 junk food comidas basura (when counting foods, Spain) comida chatarrafB2 junk food comidas chatarra (when counting foods, Latin America) dedo corazónmB2 middle finger dedos corazón dedo índicemB2 forefinger dedos índice fecha límitefB2 deadline fechas límite (also fechas límites) fecha topefB2 deadline fechas tope hora picofB2 rush hour horas pico (Latin America) hora puntafB2 rush hour horas punta (also horas puntas, Spain) navegador webmB2 web browser navegadores web palabra clavefB2 password palabras clave (also palabras claves) palabra clavefB2 keyword palabras clave (also palabras claves) papel tapizmB2 wallpaper papeles tapiz audiencia metafC1 target audience audiencias meta hombre lobomC1 werewolf hombres lobo pez espadamC1 swordfish peces espada público metamC1 target audience públicos meta público objetivomC1 target audience públicos objetivo (also públicos objetivos) temperatura ambientefC1 room temperature temperaturas ambiente coche patrullamC2 patrol car coches patrulla estado títeremC2 puppet state estados títere (also estados títeres) pez gatomC2 catfish peces gato tema tabúmC2 taboo subject temas tabú (also temas tabúes)
How it works

In these pairs the first noun is the main word. It names what the thing is: a página, a fecha, a pez, a coche. The second noun works like a label that says what kind: a web one, a limit one, a sword one, a patrol one. The main word carries the plural, and it also sets the gender. That is why it is la fecha límite and los peces espada, and why any adjective agrees with the main word: unos coches patrulla nuevos.

The same pattern shows up in classic terms like sofá cama (plural sofás cama) and camión cisterna (camiones cisterna). These are not the same as compounds written as a single word, such as abrelatas, which are built from a verb plus a noun and never change at all.

Rule of thumb: find the real thing being counted, pluralize that word, and leave the label alone.

Busca las palabras clave del texto.
(Find the text's keywords.)
La estrella está a diez años luz.
(The star is ten light-years away.)
The words
Internet and tech

Anything with web as the label follows the rule: página web, sitio web, cámara web, navegador web. The word web never gets an -s in these terms. Palabra clave means both keyword and password, and the plural is palabras clave in both senses.

Visito varios sitios web de noticias cada mañana.
(I visit several news websites every morning.)
Nunca uses las mismas palabras clave en todas tus páginas web.
(Never use the same passwords on all your web pages.)
Los dos navegadores web funcionan con estas cámaras web.
(Both web browsers work with these webcams.)
Everyday life

Fecha límite and fecha tope both mean deadline. For rush hour, Spain says hora punta and much of Latin America says hora pico. Junk food is comida basura in Spain and comida chatarra in Latin America. You will also see temperatura ambiente, papel tapiz, dedo índice and dedo corazón.

Este mes tengo tres fechas límite en el trabajo.
(This month I have three deadlines at work.)
En las horas punta el metro va lleno.
(At rush hour the subway is packed.)
Hay muchas comidas chatarra que parecen sanas.
(There are lots of junk foods that seem healthy.)
Levanta los dedos índice de las dos manos.
(Raise the index fingers of both hands.)
Animals and creatures

Pez espada, pez gato and hombre lobo all pluralize the first word: peces espada, peces gato, hombres lobo.

En el río pescamos dos peces gato.
(We caught two catfish in the river.)
En la película aparecen cinco hombres lobo.
(Five werewolves appear in the movie.)
News and formal topics

Coche patrulla, estado títere, tema tabú, público objetivo and público meta belong here.

Llegaron cuatro coches patrulla a la plaza.
(Four patrol cars arrived at the square.)
En mi familia hay algunos temas tabú.
(In my family there are some taboo subjects.)
Watch out

The typical mistake is putting the plural on the label only, as English does: página webs or año luces. Those are always wrong. Años luces and páginas webs are also wrong: pluralize the main word and stop there.

Some labels are drifting toward adjectives. When a speaker feels the second word describes the first, it can agree too. So you will meet palabras claves, fechas límites, horas puntas, temas tabúes, estados títeres and públicos objetivos, and these are accepted. This happens mostly with words that already work like adjectives, such as clave, tabú and límite. It does not happen with labels that are clearly things, like luz, lobo, espada or web. If you are unsure, the unchanging form (palabras clave, fechas límite) is always safe.

Remember that the main word also decides gender, even when the label has a different one: la hora pico (not el), la palabra clave, el pez gato.

Summary

In a noun plus noun term, the first noun is the real thing and the second is a label. Put the plural (and the gender) on the first noun and leave the label as it is: páginas web, años luz, peces espada, fechas límite. A few adjective-like labels such as clave, tabú and límite may also take a plural, but the unchanging label is never wrong.