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Excerpt from: Jo Anne Sadler, Property Marks in Rural Norway - Bumerker, 2007

Bumerker (plural) were used as property marks before people could read and write.  The practice was believed to have been in use by the Viking age and the early marks were based on the ancient Runic alphabet.  They were used to show ownership of property and as the official signature of people who could not write their name.

The mark was carved on homes, household goods, farm buildings, equipment, timber and barrels of trade goods.  To sign documents, generally a seal stamp or signet ring (segler) was made. ... To say it was the mark of an illiterate person is not entirely correct as literate people of authority, merchants, and priests would also use them as seals on documents. ...
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