Rule 18 applies between boats when they are required to leave a
mark
on the same side and at least one of them is in the
zone
However, it does not apply
between boats on opposite
tacks
on a beat to windward,
between boats on opposite
tacks
when the
proper course
at the
mark
for one but not both of them is to tack,
between a boat approaching a
mark
and one leaving it, or