A tomoe is a charge unique to Japanese Mon, a comma-shaped motif intended to represent a whirlpool in water. Tomoe are period charges, found in the Mon of Bessho Nagaharu, general and daimyo, d.1580 [Hawley 77].
The period examples of tomoe show them used in multiples, three being by far most common; always in annulo; and with no other charges in the design. This attested pattern, as in the illustration, has been accepted for Society use.
See also gout.
Samukawa Mantarou Yukimura bears: Argent, three tomoe in annulo azure.