Because this system needed 19 runes to represent the 19 golden numbers which stood for the 19 years of the perpetual calendar's cycle, the Younger Futhark was insufficient, having only 16 characters. The solution devised was to add three special runes to represent the remaining numbers: ("arlaug"; Golden Number 17), ("tvimadur" or "tvímaður"; Golden Number 18), and ("belgthor"; Golden Number 19). In 1636, Ole Worm documented the Younger Futhark numeral system, including these three characters, in his "Runir seu Danica literatura antiquissima" ("Runes: the oldest Danish literature").