In the past 111 seasons, six teams have won 110 or more games. By contrast, only two have gone through an entire year using no more than five starting pitchers.
The 2003 Mariners and 1966 Dodgers -- with Sandy Koufax, Don Drysdale and Don Sutton -- are the only clubs to pull off the latter feat. Such is the difficulty of keeping a quintet of pitchers healthy and productive enough to make it through 162 games (or fewer in past eras).
In that context, it's not surprising to hear teams talk so frequently about depth, especially when it comes to their rotations. Take the Astros' recent addition of veteran right-hander Doug Fister, whom Houston signed despite a staff that already included Dallas Keuchel, Collin McHugh, Lance McCullers, Mike Fiers and Scott Feldman.
"At full strength, we've got some options," Astros manager A.J. Hinch said. "Last year, we used 13 starting pitchers. I think the average was around 11. We're going to need every one of them."
• Fister joins Astros on one-year deal
Hinch had his facts straight. Each of the 30 teams used at least eight starters last year, with the Astros at 13 and the Dodgers leading the way with 16. Together, the clubs averaged 10.9.

So, in general, teams must have a bunch of starters at their disposal in order to get through a season. That's obvious. But just how much depth is required?
Let's go back to 1998, the year when MLB expanded to 30 clubs. That provides a sample of 18 seasons. Over that time, the greatest number of teams (107) used 10 starters, with an overall average of 10.3. Meanwhile, just 14 teams -- or not even one per year -- needed six starters or fewer. The last to do so was the 2013 Tigers.
During the same span, only six teams managed to get at least 30 starts apiece from five pitchers. That compares with 136 who had no more than one pitcher make it to 30.
Of course, the number of extra starters only tells part of the story. It would be more precise to look at how many starts those replacement arms have to make.
Since MLB added the extra Wild Card spot in each league in 2012, 40 teams have qualified for the postseason. By definition, these are successful clubs, meaning they are more likely to have fielded healthy, stable rotations. Still, most needed to dip significantly into their depth.

There are exceptions. The 2012 Reds used the same five starters -- Bronson Arroyo, Homer Bailey, Johnny Cueto, Mat Latos and Mike Leake -- for 161 games, only resorting to a sixth (Todd Redmond) for a doubleheader. That same year, the eventual World Series champion Giants, who beat the Reds in the National League Division Series, squeezed 160 starts from their top five.
But on average, those 40 playoff teams got 27 starts from pitchers who finished outside of their top five in that category. They turned an average of 39 times to those outside their projected rotation heading into Spring Training. Considering that a full workload for a starter is 32 or 33 outings, that's a substantial burden. And again, this is for successful teams.
For example, take the 2014 A's, who won 88 games and an American League Wild Card berth. Before Spring Training began, Oakland was supposed to have Jarrod Parker and A.J. Griffin in its rotation, along with Sonny Gray, Scott Kazmir and Dan Straily. But Parker and Griffin sustained season-ending elbow injuries that spring, while Straily took the ball seven times before he was demoted to Triple-A, then later traded to the Cubs in the Jeff Samardzija deal. In the end, the original quintet accounted for only 72 starts -- the lowest total among this group -- and the A's dealt for Samardzija, Jason Hammel and Jon Lester before the non-waiver Trade Deadline.
In 2015, the Cubs and Pirates were the only playoff teams to require fewer than 35 starts from pitchers outside their projected rotations. The Cardinals lost Adam Wainwright to a torn Achilles tendon in April, Zack Wheeler underwent Tommy John surgery before throwing a pitch for the Mets, and the Blue Jays' Marcus Stroman missed most of the season with an ACL injury sustained in Spring Training. The Rangers used a projected starter in only about half of their games, as Yu Darvish needed Tommy John surgery, Derek Holland battled a shoulder injury, and Ross Detwiler lost his job.

With that dynamic in mind, here is a look at five teams that stand out as having strong starting pitching depth heading into the 2016 season. This isn't a matter of the best rotations, but rather the best backup plans.
Angels: They lack certainty beyond Garrett Richards but make up for that somewhat with depth. If Andrew Heaney, Jered Weaver and C.J. Wilson are behind Richards, that leaves only one spot for 2014 AL Rookie of the Year Award runner-up Matt Shoemaker, who struggled last season; 2015 AL All-Star Hector Santiago, who slid badly in the second half; 25-year-old Nick Tropeano, who posted a 3.93 ERA and 3.8 strikeout-to-walk ratio in seven starts last year; and 24-year-old lefty Tyler Skaggs, who by Opening Day will be almost 20 months removed from the Tommy John surgery that interrupted his encouraging '14 campaign.
Astros: If Fister is in the rotation, then Feldman or Fiers likely isn't, though the Astros also could limit McCullers' innings a bit. Straily and Brad Peacock provide further depth, along with prospect Asher Wojciechowski. Houston also signed veteran lefty Wandy Rodriguez to a Minor League deal.
Cardinals: Even with Lance Lynn out for the year, the Mike Leake signing filled out the rotation. That leaves three young left-handers -- Tim Cooney, Tyler Lyons and Marco Gonzales -- either in the bullpen or waiting in Triple-A. Each has some big league experience, with Cooney and Lyons combining for a 3.70 ERA in 14 outings last year.
Dodgers: Losing Zack Greinke hurts, but they retained Brett Anderson while adding Scott Kazmir and Kenta Maeda. With Hyun-Jin Ryu expected back for Opening Day, Alex Wood (3.31 ERA in 66 career starts), Mike Bolsinger and Carlos Frias could be in the bullpen or at Triple-A. Brandon McCarthy is expected back from Tommy John surgery sometime midseason, and Brandon Beachy will be a non-roster invitee this spring. No. 4 prospect Julio Urias leads a group of young arms that also could be ready to contribute this year.
Nationals: They took a chance on Arroyo with an incentive-laden Minor League deal and also signed former Giants righty Yusmeiro Petit, who has plenty of experience in a swingman role. Taylor Jordan has made 15 starts for the Nats over the past three seasons, and A.J. Cole remains a highly regarded young arm. Lucas Giolito, the game's top-rated pitching prospect, also is waiting in the wings.