Drop D down a 1/2 step: From standard Drop D tuning,
tune all of the strings down a 1/2 step (like they were1 fret
lower)
Drop D down a whole step: From standard Drop D tuning,
tune all of the strings down a whole step (like they were 2
frets lower). This is sometimes called Drop C by mistake. Drop
C would mean to drop only the 6th string down to a C.