A calendar is a leap month calendar if: * it divides time into years which are more-or-less in sync with seasonal (a.k.a. tropical) years, * it divides years into months (which may or may not correspond with lunations), * most years have a fixed number of months but * in some years there is an additional month which is added in order that calendar years stay in sync with seasonal years. The extra month (which is not always the 13th month) is called either a leap month or an intercalary month (due to intercalation within the other months).