Prosodic words can be grouped in turn into larger units, prosodic phrases defined by rhythm not strictly by grammar. Alliterative verse is based very closely on the prosodic word and the prosodic phrase. The strong, alliterating stresses of alliterative verse must be the strong, central stress around which a prosodic word is built. It is the root stress of a content word, typically a noun or adjective. It is the head stress of a prosodic word.
Strong stresses have to be lifts, and typically alliterate.
We shall see shortly how these rules are applied.