Officially (going by the
indicia at the bottom of the first inside page of the
comic), the title changed to
Uncanny X-Men with issue #142; prior to that, the title was just
X-Men with no adjective. However, "
Uncanny" had been used on the front cover above the X-Men logo for some time. (Still earlier, "All New, All Different!" had been used, to distinguish the
New X-Men from the older incarnation prior to issue #94.) Once "Uncanny" became part of the official title, that freed
Marvel to create yet another X-Men title
without the adjective, which they did with great fanfare during the "
Gimmick Age" of the early '90s (multiple variant covers and all). Use of adjectives in the titles of comics is a long-standing
Marvel tradition (
Amazing Spider-Man,
Incredible Hulk,
Fantastic Four), but publishing different titles simultaneously with and without the adjective is a
1990s gimmick.
Some price guides, checklists, and other comics references split the title at the point of the official change, with earlier issues under "X" and later ones under "U"; this results in the famous and influential 2-part continued story "
Days of Future Past" (in issues #141 and #142) getting split between the two listing sections.