The original marks for scansion came from the quantitative meter of classical prosody where long syllables were marked with a macron( ¯), and short syllables were marked with a breve ( ˘).
˘ ¯ ˘ ¯ ˘ ¯ ˘ ¯ ˘ ¯
But SOFT! What LIGHT through YONder WINdow BREAKS?