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In Sabermetrics, the defensive spectrum is the graphical representation of the positions on a baseball field, arranged from top (the easiest defensive positions) to bottom (the hardest).

The defensive spectrum looks like this:

1. Designated hitter
2. First baseman
3. Left fielder
4. Right fielder
5. Third baseman
6. Center fielder
7. Second baseman
8. Shortstop
9. Catcher
10. Pitcher

Like many original sabermetric concepts, the idea of a defensive spectrum was first introduced by Bill James in his Baseball Abstract series of books during the 1980s. The basic premise of the spectrum is that positions at the bottom end are more difficult than the positions at the top end of the spectrum. Therefore, the positions at the top are easier to fill, since the physical demands are less as you move left along the spectrum. A corollary to this is the fact that, since defensive skill is at less of a premium at the top end, players at those positions must provide more offense than those at the bottom end. Another corollary is that players can generally move from bottom to top along the spectrum successfully during their careers.

The defensive spectrum has shifted only once in the 130 year history of baseball. In the late 1800s and early 1900s, third base was generally considered to be more challenging than second base. This was because the double play was relatively uncommon in this period; thus, the third baseman, who had to field hard hit grounders and throw the ball 120 feet to first base, had a far more challenging job than the second baseman, who threw the ball 70 feet at most. The result of this was that there were far more good hitters at second base than third; great-hitting early second baseman include Nap Lajoie and Rogers Hornsby. This defensive spectrum looked something like this:

1. First baseman
2. Left fielder
3. Right fielder
4. Second baseman
5. Center fielder
6. Third baseman
7. Shortstop
8. Catcher
9. Pitcher

However, by the 1920s and 1930s, the defensive spectrum was beginning to shift. Double plays were becoming steadily more common, increasing the defensive responsibilities of second base. Offense was therefore increasingly important at third base. One of the first new third basemen was Harlond Clift of the St. Louis Browns, who was notable as one of the first third basemen to hit 30 home runs. By 1945, second base was firmly established as a more defensively important position than third base.

• The Bill James Primer
• Baseball Prospectus: "Rethinking the Defensive Spectrum"