Two-word set phrases arise during the generative formation of English nouns. Such set phrases sent an early stage in the process of noun compounding, signalled solely through word stress. In English, the rules for noun stress place stress away from the end (i.e., not on last syllable). Following this pattern, compound nouns in English receive stress on the first word in the compound, not the last: something, greenhouse, mousetrap.
The set phrase is a compound noun in-the-making. Two words become fixed to mean one thing or idea. As written, a space separates each word in a set phrase (e.g., the white house, a moving van). Such a space usually signals the boundary between words. When found in a set phrase, however, the space introduces ambiguity; for instance, it is unclear whether the white house refers to "a white house or a sexy man" or "the White House". When the set phrase is uttered aloud, it is articulated more like a compound noun, not two distinct words: the white house, a moving van. This pattern follows the normal rule for stressing English nouns and English compounds. In addition, the shift in stress mandates a change in the spelling of "white house" to "White House." Orthography is dictated by stress.
Set phrases often have minimal pair counterparts composed of an unstressed adjective + STRESSED NOUN. This pattern is the default. Using the adj+NOUN stress pattern makes the meaning more general (e.g., a white house or a moving van), or changes it entirely (e.g., a computer screen [the monitor] vs. a computer screen [a data screen performed by computer]).
The observation that stress alone can turn adj+NOUN combinations into full-fledged nouns suggests that compounding is first notated in English through prosody. After achieving widespread usage and acceptance, the two-word combination, spoken as a set phrase, becomes fixed as a compound word and spelled as such.
Base words that have spatial or container semantic content frequently combine as set phrases: room, office, box, bag.
Action-based "ing" words when combined with nouns representing spaces and/or containers frequently trigger set phrase stress.