We all know using a designated hitter helps the offense, but I rarely think about how much it helps the defense. Obviously pitchers are used more optimally when there’s a DH in the lineup, because they don’t need to be removed for pinch hitters. More significantly, I would say, a team can use the DH slot to play a better defender than one of the regular eight, who is moved to DH. How significant? Well, the first step in finding out might be to see how often that is done. Perhaps, we will discover something else in the process.

In the American League, we can see how often the regular DH moves into the starting line-up when playing a National League city. We can fairly suppose that is a reasonable percentage of the time the A.L. team is improving its defense when it uses the DH in A.L. cities. The more often a team’s DH plays in his line-up against N.L. cities, the more likely he’d be playing against A.L. cities, if there was no DH. So, when he is DHing, his team is improving its defense by moving a better fielder to his position.

Below are the number of times the regular DH started the game in N.L. ballparks over the number of games in N.L. ballparks up through August 19, 2015. I carefully checked for platoons, injuries, and line-ups changes to choose the most appropriate "regular" DH at the time of the interleague game.

That’s 40 of 118 N.L. games the regular DH started the game as a fielder - about a third of the games (34%).

More directly with the National League teams, we can see how often an expected regular position player is moved to DH in an American League game. No N.L. club had a full-time DH, although, Ryan Howard of the Phillies, Michael Cuddyer of the Mets, Kyle Schwarber of the Cubs, and Willin Rosario of the Rockies might qualify.

As of August 19 inclusive, here are the number of games a regular position player was the DH over the number of games in American League ballparks:

As you can see, National League teams have used the DH almost twice as often for a defense upgrade - or, at least, for resting one of their starting eight fielders by having him DH. So far in 2015, National League teams have done this in 85 of 130 occasions or 65%. The Dodgers and Rockies are major exceptions. Without them, the use of a regular fielder as the DH would be 71%.

By the way, the Mets were the most difficult team to say who are the regular starters. I counted Cuddyer’s early season DHing as a regular eight having moved, but his most recent appearance there as a substitute adding his bat.

I’m not sure why there is such a difference between leagues. Sure, it makes sense that American League teams would have a professional DH employed who wasn’t fit for the field. However, there is no significant correlation of the teams not putting their DH on the field in N.L. cities to having a professional DH.

The teams with the most dedicated DHs—Yankees, Red Sox, Royals, Tigers, White Sox, Astros, Rangers, Athletics, and to some degree the Blue Jays and Orioles—actually have a higher rate of moving their DH to the field (42%) than the teams with the least settled DHs (Rays, Twins, Indians, Angels, and Mariners), who averaged only 22% of the DHs starting in the field against N.L. cities. Move the Orioles over to the mix & match bunch as their DH spot was very unsettled at the start of the year until Jimmy Paredes fell into it and the percentages gets closer: 39% to 29%.

I am grateful that the two leagues haven’t agreed on the DH, so that we study the difference.