Plural: "runners up" or "runner ups"?

Plural: "runners up" or "runner ups"?
 

I have a problem with plurals when it is a double word or a compound word joined using hyphens. I suppose it is brothers-in-law though people use to call in-laws to their idem.

Is there a general rule?

runners up
brothers-in-law
passers-by

The base word, or more significant, word is what is made plural.
When both words are equally important, like in grown-up, the 's' goes on the end.

Words like teaspoonful and truckful have two accepted plurals. Placing the 's' on the end is more prevalent, however, and is the newer form.

Yep, but you would be surprised at how many people say it wrong. I guess maybe it feels more natural to say brother-in-laws, but you have multiple brothers and not multiple by-law families. If that makes sense. Nice explanation Rusty!:good:

I basically agree with Rusty, with one additional note: in the compounds which mark the plural on the main word instead of the last word, the main word is usually a noun: (brother-in-law -> brothers-in-law, teaspoonful -> teaspoonsful, truckful -> trucksful, man-of-war -> men-of-war, plus many military and diplomatic terms borrowed from French such as court-martial -> courts-martial), However, when the main word is NOT a noun, then usually the plural suffix goes on the end of the compound (grown-up -> grown-ups).

The possessive suffix is a different matter: that always attaches to the end of the compound, even when the plural is formed elsewhere: brother-in-law, brothers-in-law -> brother-in-law's, brothers-in-law's.

Thank you very much, all of you.

Then, plural mark on nouns or in the end if coming from adjectives, phrasals, etc

Make-ups(1), take-aways(2), tic-tac-toes(3) and go-betweens(4), but attorneys-at-law(5), I-beams(6), bull's-eyes(7), editors-in-chief(8) and merry-go-rounds(9). Jacks-in-the-box(10) or jack-in-the-boxes, and other few exceptions. And definitively, runners-up (11)

1- maquillajes || confecciones || etc.
2- recuperaciones (sports)
3- ta-te-tis / tres en raya / tres en línea
4- intermediarios
5- abogados
6- vigas doble-T / perfiles doble-T / vigas I / perfiles I
7- dianas / centros de blanco
8- editores en jefe
9- tiovivos / calesitas
10- muñecos de resorte / cajas de sorpresa
11- segundos / subcampeones

I don't think make-up is a count noun.

I think you are right, at least about some meanings, like "maquillaje". I found some corpus cases that apparently make sense:

"... marriages and near divorces, our own fights and make-ups..." (reconciliaciones||avenencias)
"... we have different make-ups, we have different genes... " (constituciones||contexturas)
"... so many people are talking about the racial make-ups of the two juries ..." (composición ---> uncountable in Spanish)
"... has started a new individual portrait competition. First judging will be in April. If you are a little too late, make-ups will be allowed,..." (nuevas instancias || repechajes)

All four cases sound to me like very low register. In each case, there is a better word.

'Makeup', without the hyphen, is more common for 'maquillaje'. I didn't think 'makeup' was a countable noun, either, but discovered that some people on the internet think so (like, "I've tried various makeups, lotions, ...").

The hyphenated word 'make-up', with the meanings you noted, may be pluralized. I've personally heard and used each one of the plural forms you posted.

By the way, the word 'people' is plural in English. "So many people are ..." :)

Thank you!
Oops! I've change it (It's just lower the guard one second and my Spanish brain takes control)