Words like




are used mingled, the wheel goes with barrow in inseparable form.

On the other hand, words like

tank top


high heels

as it sounds written separable.

  1. On what reasons they're written like this?
  2. Is there a clear way to distinguish the correct form and making them in writing?
  • "Wheelbarrow" is one word. "Nailbrush" is apparently recognized by some dictionaries, but is not commonly used. – Hot Licks Jan 7 '16 at 2:05

Yes, there is a way to tell: Look in the dictionary. I'm not trying to be glib. I look in the dictionary all the time to check to see if something is two words or a single compound word. In fact, earlier today I couldn't remember if "more so" was one word like "notwithstanding." It's not, but to be sure, I had to check.

Anyway, there is no set pattern you can fall back on to tell. Even the same word goes many different ways: halftime, half-day, half past. All three refer to time; all three combine "half" differently. Often, the same construction deviates from itself: mid-twenties and late twenties. This makes the phrase "mid- to late twenties" an awkward construction. The long and the short of it is it's without rhyme or reason, so you either have to try and remember it all or keep a dictionary handy—or more likely, both.

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