Pentimal System

The pentimal system (Swedish: pentadiska siffror) is a notation for presenting numbers, usually by inscribing in wood or stone. The notation has been used in Scandinavia, usually in conjunction to runes.

The notation is similar to the older Roman numerals for numbers up to 9 (I - VIIII). Unlike the Roman notation, the notches are placed vertically on the stem or stav of the rune. The number 4 is represented by four horizontal lines on the stem, 5 is represented by what looks like an inverted letter U. 10 is represented by two U's opposing each other. Numbers up to 19, or even 20, can be represented by a combination of I's and U's, just like the Roman numerals are represented by combinations of I's and V's. (A Roman 10 is represented by two V's opposing each other).

The widest use of the notation is in presenting the Golden Numbers, 1 - 19 on Runic calendars (Danish: kalenderstave, Swedish: runstavar, Norwegian: kalenderstavar, also known as clogs). The numbers are commonly found in Modern Age and possibly Early Modern Age calendar sticks. It is unknown if they were in use in the Middle Ages, let alone in the Viking Age. On older runic calendars, a different notation for representing the Golden Numbers was used; the 16 runes of Younger Futhark represented the numbers from 1 to 16 and three ad hoc runes were used for the numbers 17 to 19. For example, the Computus Runicus manuscript, originally from 1328, but collected and published by the Dane Ole Worm (1588-1654), uses this futhark notation, and not the pentadic numerals under discussion here.