Apple has held to the n-2 rule, although the timing of an edition's final update has varied.

Last year, Apple shipped the last OS X-wide security update for Leopard (n-2), the edition released in Oct. 2007, on June 23, 2011, almost a month before the launch of Lion (n). OS X Tiger (n-2), which appeared in April 2005, received a final security update on Sept. 10, 2009, 12 days after Snow Leopard (n) shipped.

Prior to that, Panther (n-2), or OS X 10.3, got its final update two-and-a-half weeks after the debut of Leopard (n), while OS X 10.2, aka Jaguar (n-2), saw its last patch three months before the launch of Tiger (n).

An edition's retirement isn't comprehensive. Apple has typically delivered separate security updates for four components: iTunes, Java, QuickTime and Safari for several months beyond the cut-off.

Apple usually delivers the final security update for an older edition ('n-2') around the time when a new upgrade ('n') ships. (Data: Apple.)