American English -- Compound Nouns -- Page 1
Explanation and Discussion In American English Compound Nouns are sometimes hyphenated and sometimes not. No real rules exist but some trends do. Below I have some of the major groups of words and their generalities.

Up Words Some compound nouns ending in "up" are either solid or hyphenated. For Example:
backup blowup breakup buildup checkup cleanup getup holdup letup
lineup linkup makeup markup paste-up pileup roundup setup slipup
smashup warm-up windup call-up catch-up close-up cover-up flare-up follow-up
foul-up hang-up jam-up lead-up mix-up mock-up runner-up send-up shake-up
sign-up start-up tie-up toss-up touch-up wrap-up write-up

Down Words Most compound nouns ending in "down" are solid. For Example:
breakdown closedown comedown countdown crackdown letdown lowdown markdown meltdown
rubdown rundown shakedown showdown shutdown slowdown sundown turndown dressing-down
put-down sit-down step-down wind-down write-down