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.
For a few compounds, there may be two or three acceptable forms.
- open compound (separated by a space)
- hyphenated compound
- closed compound (no separator)
- clearing-house, clearing house, clearinghouse
- cop-out, cop out, copout
- end user, end-user
- get-together, get together
- round table, round-table, roundtable
- second hand, second-hand, secondhand
- touch screen, touchscreen
- trade-off, tradeoff [substitute-off? exchange-off?]