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The Spectrum 
The defensive spectrum is:
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 on the right side of the spectrum are more difficult than the positions on the left side. Therefore, the positions are easier to fill as one goes left on the spectrum, since the physical demands are less. A corollary to this is that, since defensive skill is at less of a premium on the left side, players at those positions must provide more offense than those on the right. Another corollary is that players can generally move from right to left along the spectrum successfully during their careers, but moving a player from left to right is quite risky.
In some versions of the defensive spectrum, pitcher and catcher are not included, since certain defensive demands of those positions are so specialized as to be inapplicable to players at other positions. The designated hitter is sometimes omitted since he is technically not part of the "defense" at all.
As an example of the concept in action, players who are drafted by Major League Baseball teams as shortstops are far more likely to ultimately end up at a different position than players who are drafted as first basemen.
Shifts in the Defensive Spectrum 
A retrospective analysis of the 130-year history of baseball shows that the defensive spectrum has shifted only once. In the late 19th century and early 20th century, 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. Frequent bunting also meant that the third baseman would be more often challenged defensively.
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 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.