In historical materials, dates will often be indicated with what appears to be duplicate, or excessive digits, sometimes separated by a hyphen or a slash. This is often referred to as double dating. The need for double dating arose from the transition from an older calendar to a newer one. For example, in "10/21 February 1750/51", the dual day of the month is due to the leap year correction of the Julian calendar by the Gregorian calendar, and the dual year is due to some countries beginning their numbered year on 1 January while others were still using another date.