Languages evolve. Words can shift from between: two totally unrelated words, collocation, open compound, hyphenated compound, or a closed (or "solid") compound.

The separators can usually be added or removed without too much confusion, though at any instant in time, there's usually only one acceptable form.

  1. open compound (separated by a space)
  2. hyphenated compound
  3. closed compound (no separator)
For a few compounds, there may be two or three acceptable forms.

See also