Penny Napoleonic Wars prompted the use of base metals in 1797. Despite the decimalization of currencies in the United States and, later, throughout the British Commonwealth, the name remains in informal use. No penny is currently formally subdivided, although farthings (¼ d.), halfpennies, and half cents have previously been minted and the mill (1/10¢) remains in use as a unit of account in some contexts. "Penny" is first attested in a 1394 Scots text, a variant of Old English ', a development of numerous variations including ', ', and '. The etymology of the term "penny" is uncertain, although cognates are
Penny graph coin graphs are the same as the planar graphs, all penny graphs are planar. The penny graphs are also unit disk graphs (the intersection graphs of unit circles), unit distance graphs (graphs that can be drawn with all edges having equal lengths, allowing crossings), and matchstick graphs (graphs that can be drawn in the plane with equal-length straight edges and no edge crossings). The Hanoi graphs formula_1 are penny graphs. Every vertex in a penny graph has at most six neighboring vertices; here the number six is the kissing number for circles in the plane. However, the pennies whose centers