Norwegian house mark with a variation of the shape hourglass.
House mark is originally a mark of property, later also used as a family or clan emblem, incised on the facade of a building, on animals, in signet and similar in the farmer and burgher culture of Germany and Scandinavia.
The purpose of a house mark is to have a recognisable mark that a person, a nuclear family, multiple generations of an extended family or an owner of a property can use to mark objects, cattle or buildings for recognition of ownership. The use of house marks dates back to long before writing was public knowledge.
The Norwegian word bomerke or boemerke probably came from Denmark. There is no Norwegian reference before the 17th century. Today bomerke is mainly written as bumerke in Norwegian. Both in Denmark and Sweden, the word bomerke (with multiple spellings) is used since the 14th century and in the 16th century. In the Icelandic codes of law from the Middle Ages, one finds the word einkunn used to denote owner marks used to tag animals. It is likely that this word has also been used in Norway. 
The form of house marks is based on function. They should be easy to cut, scratch or engrave with a knife or similar tool. At the same time they should be distinctive and easy to remember.
House marks can be made from one or two lines and up to quite a complex pattern of line figures. Based on appearance house marks resemble line figures in rock carvings and in early writing systems. It is unclear how extensive such ancient line figures were used as marks for people or property ownership.
The basic forms of a house mark is often runes, characters and numbers, stylized figures, international symbols like crosses, stars, and astrological or astronomical characters.
One characteristic of house marks is that they may consist of a basic form with addition or deduction of lines. In this way related people can have marks that resemble each other, but differ by details. This is equivalent to cadency and adding brisures as methods to change the coat of arms.
Many house marks are placed in the shield-shaped frames. We see this in seals, on buildings and on tombstones, for both farmers and city dwellers in Scandinavia and German areas, during the 1700 and 1800 century. Some of these house mark shields also has color and is approaching the heraldic coat of arms.
Allan Tønnesen: Bomerker og runer, Heraldisk Tidsskrift 51/23, København 1985
Allan Tønnesen (editor): Magtens besegling. Enevoldsarveregeringsakterne af 1661 og 1662 underskrevet og beseglet af stænderne i Danmark, Norge, Island og Færøerne, Syddansk Universitetsforlag, Odense 2013, 583 p., ISBN 9788776746612.