A fractional
rig is when the forestay (and therefore the
jib) doesn''t go all the way to the top of the mast, but only part way. For example, a 7/8
rig would have the forestay run to about 1/8 of the way down from the top. On masthead
rigs, the stay goes to the top, and you get a bigger
jib &
spinnaker. Lately some
rigs use fractionnally rigged
jibs, but still fly
spinnakers from the top of the mast for better downwind performance.