A legacy numbering refers to the accumulated number of issues of a given series which has been relaunched one or several times. When a series is relaunched, its issue number is reset, and the legacy number keeps track of the overall number of issues. Although legacy numberings were first used in early 2000s, the term was coined with 2017's Marvel Legacy.

In most cases, rather than keeping track of just the overall issue number of a series, a legacy numbering will account for the overall issues of a character's main title. For instance, the character of Moon Knight has starred in the titles Moon Knight, Marc Spector: Moon Knight, and Vengeance of the Moon Knight, all of which have been the character's only title at the time, so Moon Knight's legacy numbering takes the number of issues of those series as well. This is not the case for characters with concurrent titles, like Spider-Man. His main series, Amazing Spider-Man, has one legacy numbering, and its sister title Peter Parker: The Spectacular Spider-Man had its own.

It should be noted that the counting of legacy numbers includes a factor of arbitrariness. The number of issues of the mini-series Avengers No Road Home were added to the legacy numbering of Avengers despite the two series being concurrent; the legacy numbering of Miles Morales: Spider-Man includes issues starring his predecessor, Peter Parker; the legacy numbering of Iron Man originally didn't include three issues of Iron Man: Director of S.H.I.E.L.D. since they starred War Machine, but those issues were later on included; and the initial run of Superior Spider-Man accounted for the legacy numbering of both Amazing Spider-Man and Superior Spider-Man (Vol. 2).

Legacy numberings were first used in 2001, a few years after the relaunch of many titles due to Heroes Return. The legacy numbering was presented as a secondary element in the trade dress of a comic's cover, featured under the series' main issue numbering. This practise was phased out by 2004. In some instances, a series would return to its legacy number for a landmark issue. For example, Amazing Spider-Man (Vol. 2) #58 corresponded to #499, so the series resumed its legacy numbering for the next issue, Amazing Spider-Man #500. Legacy numberings were abandoned in 2013 with the line-wide relaunch of Marvel NOW!, and were briefly reintroduced as the main numbering of most series for Marvel Legacy. Series were relaunched en masse afterward for Fresh Start, but legacy numberings returned to being featured as a series' secondary numbering, appearing once again under the series' main issue numbering, with the added prefix "LGY."