When it comes to linking words to form new words, English is a particularly mischievous language. Different compound words with an element word in common, or pairs of words analogous to each other, may be inconsistent about the presence or absence of a letter space or a hyphen when you see them listed in the dictionary and other resources. That goes especially for directional duos. Keep a sharp lookout for these spacing shenanigans:
Front and Back
For some perverse reason, a few common compounds that include front, and their back correspondents, are treated differently: “front door,” backdoor (but only as an adjective); “front seat,” backseat; “front yard,” backyard. How could this have happened?
Perhaps it’s the ubiquity of other closed compounds beginning with back (such as backache, background, and backlash) compared to the absence of front-loaded analogues. Speaking of front-load, compounds beginning with front, such as that word and “front man,” are invariably open or hyphenated, and if they have back counterparts (you can back-load, but no one refers to a back man), those are also open or hyphenated.
Another contributing factor may be that back constructions are idiomatically richer: “backdoor man,” “backseat driver,” and “backyard grill” have given compounds beginning with back a higher profile, so it’s likely they tend to evolve from open to closed compounds with greater alacrity — becoming front-runners, as it were.
In and Out
After studying compounds beginning with in, I’m done in. Adjectival forms, whether tangible (in-flight) or intangible (in-depth), are often hyphenated, but so are many noun forms, such as in-group and in-joke. Meanwhile, most hyphenated terms beginning with out are obscure, like the fiscal term out-year, or as with out-migrate (“emigrate”). Exceptions include out-front, meaning “honest,” and out-there, meaning “unusual.”
Fortunately, the most common usages are inbounds (though the antonym for that word is not outbounds, but “out of bounds”). Indoor and outdoor, inward and outward, inset and outset (though that last pair do not have antonymic meanings) don’t try to outfox or outbox us.
But speaking of outbox, why, in clerical contexts, is it in-box and out-box, not inbox and outbox? This reasoning is a stretch: Though you can’t inbox someone, you can outbox them, so that form’s already taken. I’m satisfied to see the clerical terms remain hyphenated, while the pugilistic outbox is closed.
Up and Down
Why do you show up for a showdown? In this case, one is a verb phrase and the other is a noun. But compound nouns ending with up are usually closed (buildup, markup, windup). An exception for closure is close-up (meaning “a proximal view,” not “to lock a store for the night,” which would be hyphenated only before a noun: “He carefully followed the close-up procedure”). That’s because it follows the rule that words ending with vowels are generally hyphenated to others, rather than, well, closed up.
Adjectival compounds beginning with up (such as up-country and up-tempo) and down (generally, more obscure than their up counterparts, like down-home) tend to be hyphenated. However, up nouns are usually closed (upshot, downfall). Though open or hyphenated up equivalents are almost nonexistent, down compounds can be open (the card-playing term “down card”) or hyphenated (the music term down-bow) as well.
What’s the take-away (not to be confused with take-in)? Keep your dictionary handy.