Latin, "for the sake of the
meter" (see
metrics). Since Latin
poetry followed such demanding
metrical rules,
poets often had to resort to all sorts of tricks to make their lines "fit the meter"; "metri causa" is an excuse for what might otherwise seem a
defect or
non sequitur in a poetic work. The poet
had to do that -- for the sake of the meter!
Sometimes this would even extend to changing the plot of a narrative poem.