So what is a2b? How is it calculated? More importantly, why is it calculated? One question answered at a time.
Specifically, a2b is the number of doubles hit for a player if he were playing in the American League in a neutral park in 1980. A2b is similar to EQA or OPS+ in that it is adjusted to account for the context of the year and the ballparks of that time.
Important differences in the calculation, however, are that it accounts for doubles alone, and no other offensive factors. This leads to some different park factors (best all time doubles park: Fenway park for righties), and year adjustments, which are available from the always estimable www.baseball-reference.com. Because of the vast differences of modern parks (you know, like the customary outfield wall), I only used years from 1940 onward.