9 mm wide -
A Viking ring is initially a symbol of the blacksmith's skills that appear in Nordic mythology.

Knots and snarls without ends were a symbol of life with its eternal change and of central importance to the Vikings.

In Viking times, it was common to swear allegiance to one's chief through various ceremonies:
In the sword-holding ceremony, the chief held the sword above his knees, and the one who was to be in service sat in front and placed his hand on the sword-hold. 
In another ceremony, the servant received a gift from his chief, who was now indebted to the giver.
The debt was paid through loyal service. After these ceremonies, a gold ring was usually exchanged, a symbol of the covenant between the chief and his servant.
With the ring, the oath was locked into an eternal circle.

In addition, rings, originally often bangles, were used as ornaments and as a means of payment.

How to find the right ring size