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Dactylic tetrameter is a metre in poetry. It refers to a line consisting of four dactylicfeet. "Tetrameter" simply means four poetic feet. Each foot has a stressed syllable followed by two unstressed syllables, the opposite of an anapest, sometimes called antidactylus to reflect this fact.
Example
A dactylic foot is one stressed syllable followed by two unstressed ones:
DUM
da
da
A dactylic tetrameter would therefore be:
DUM
da
da
DUM
da
da
DUM
da
da
DUM
da
da
Scanning this using an "x" to represent an unstressed syllable and a "/" to represent a stressed syllable would make a dactylic tetrameter like the following: