How did we, at Baseball Prospectus, predict that the Rays, who had never won more than 70 games in any season, would approach 90 wins last year? We recognized that they had moved the right players into the right positions. The result was perhaps the most impressive defensive turnaround in baseball history. According to our Fielding Runs Above Average (FRAA) metric, which measures the runs saved or lost relative to the MLB average, Tampa Bay improved from –117 to +59. Most likely, no team will make that big a jump this season. But there are some defenses we're projecting to be significantly better or worse as a result of personnel moves, reversion to the mean or some other factor. And as the Rays proved, changes in D can lead to changes in the win-loss column.