Martin's Standard Series was formally introduced in 1996, but the majority of standard Martin guitars built before the mid-1980s appear in this series. As Martin began expanding into limited editions, special editions, vintage reissues, and other "non-standard" guitars, they realized that they needed a series for their regular guitars to appear in. The Standard Series continues traditional building methods with various modern technologies mixed in. There really was no change in specifications or construction when the Standard Series was introduced, aside from the designation of the Standard Series.
The Standard Series encompasses several models and body styles, so they are broken out into each body style numerically/alphabetically and then listed numerically within each sub-category.