Figure 5-29.IALA Maritime Buoyage System cardinal marks.
flashing followed by a long flash every 15 seconds (Qk
 + LF1 15s)
WestVery quick flashing every 10 seconds (Vqk
 10s) or quick flashing every 15 seconds (Qk 
As a memory aid, associate the number of flashes
in each group with a clock face (3 o'clockeast, 6
o'clocksouth, and 9 o'clockwest).
The long flash immediately following the
group of flashes of a south cardinal mark is to
ensure that its six flashes cannot be mistaken for
three or nine.
Quick flashing lights flash at the rate of either 60
or 50 flashes per minute. Very quick flashing lights
flash at the rate of either 120 or 100 flashes per
minute. It is necessary to have a choice of quick
flashing or very quick flashing lights to avoid
confusion. Two north buoys that are placed near
enough to each other to be mistaken is one example
where the quick flashing and very quick flashing
lights would be needed.