Packard also had one other advantage that some other luxury automakers did not: a single production line. By maintaining a single line and interchangeability between models, Packard was able to keep its costs down. Packard did not change cars as often as other manufacturers did at the time. Rather than introducing new models annually, Packard began using its own "Series" formula for differentiating its model changeovers in 1923 borrowing a stragety from GM called planned obsolescence. New model series did not debut on a strictly annual basis, with some series lasting nearly two years, and others lasting as short a time as seven months. In the long run, though, Packard averaged around one new series per year. By 1930, Packard automobiles were considered part of its Seventh Series. By 1942, Packard was in its Twentieth Series. The "Thirteenth Series" was omitted.