Isolated danger marks

An Isolated Danger mark indicates an isolated danger of limited extent, which has navigable water all around it. This danger might include rock, a wreck or an isolated shoal.
An Isolated Danger mark has a shape of a pillar or spar, and is black with one or more broad horizontal red bands.
As a top mark it has two black spheres, one above the other, clearly separated.
The Isolated Danger mark exhibits a white light flashing in pairs.
To memorize the light rhythm:
Associate the group of two flashes
with the two black spheres.