If a dime passes thru the ring opening tightly, the ring is a 7.5.
If a penny passes thru the ring opening tightly, the ring is a 9.5.
If a nickle passes thru the ring opening tightly, the ring is an 11.5.
There are also approximations. If a dime, for instance, passes thru loosely but a penny won't go thru, approximate that the ring is around an 8-9.
If a buyer doesn't advertise the ring's size, ask them, "does a dime, penny or nickle pass through the opening?" Once they respond, you'll have a very good idea about what is the size of the ring they are offering.
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