Most English compounds have a head. This means that these compounds have a central element which could , at least theoretically, replace the whole compound regarding the compound's distribution within a sentence. So the head determines the word class of the compound. If the head is a noun, the compound is a noun as well. If the head carries a plural morpheme, the whole compound is regarded as a plural, too. The same is true for verbs and their inflection. In very clear-cut cases, the left-hand constituent of the compound clearly modifies the head of the compound on a semantic level as well. In example (1), a blackbird is a special kind of bird, for instance.
Examples:
(1) blackbird
N'
A N
black bird
(2) overreact
V'
P V
over react
(3) airports
N' [+ pl]
N N [+ pl.]
air ports
Even though most English compounds follow the RHR, there are also a few left-headed compounds.
They are usually based on a prepositional or phrasal verb, e.g. passer-by (based on to pass by) or even a whole phrase, e.g. mother-in-law.
Example:
(4) passer-by
N'
N P
passer by