For example, in the big red dog, the word dog is the head, as it determines that the phrase is a noun phrase. The adjectives big and red modify this head noun. That is, the phrase big red dog is a noun like dog, not an adjective like big or red. Likewise, in the compound noun birdsong, the stem song is the head, as it determines the basic meaning of the compound, while the stem bird modifies this meaning. (That is, a birdsong is a kind of song, not a kind of bird. If bird were the head, the order would be different: a songbird is a kind of bird.)
However, these simple dichotomies run into problems when they are extended, as they often are, to describe the entire language rather than specific areas of its grammar. For one, many languages are not consistently either head initial or final, or head or dependent marking, across different aspects of their grammar; and secondly, it is difficult find a definition of 'head' that is consistent across these different aspects. For instance, either the subject or the verb may be considered the 'head' of the clause in different theoretical treatments, resulting in alleged descriptions of languages as 'head initial' or 'head marking' that are theoretical claims rather than actual descriptions.
The ˈbus was late.
The low head is the syllable which begins the head and is low in pitch, usually lower than the beginning pitch of the tone on the tonic syllable.
The ˌbus was late.