and the other more angular (closer to a rectangle). A further distinction is made in falsification-hindering typeface as used on German car number plates by slitting open the digit 0 on the upper right side. In some systems either the letter O or the numeral 0, or both, are excluded from use, to avoid confusion. Year label In the BC calendar era, the year 1 BC is the first year before AD 1; there is not a year zero. By contrast, in astronomical year numbering, the year 1 BC is numbered 0, the year 2 BC is numbered −1, and so forth. See also Brahmagupta Aryabhata Division by zero Grammatical number Gwalior Fort Mathematical constant Number theory Peano axioms Signed zero Notes References Bibliography Historical studies External links Searching for the World's First Zero A History of Zero Zero Saga The History of Algebra Edsger W. Dijkstra: Why numbering should start at zero, EWD831 (PDF of a handwritten manuscript) Elementary arithmetic 00 Indian inventions