Friday is September 1, which means all 30 MLB teams are now free to expand their active rosters from 25 players all the way to 40 players, if they choose. Most teams do not play with a full 40-man active roster. Most carry 30-35 players, and there is no rule against teams having different roster sizes for a game or a series. If one team has 28 players and another has 37, so be it.

September call-ups have been around forever, though these days more and more people -- by people I means fans, reporters, analysts, and even team executives -- are speaking out against them. They don't like that the last month of the season, the most important games of the year, are played with extra players. It seems inevitable that a rule change will be made at some point, limiting the number of active players per game in September.

Me? I'm pro September call-ups. I think they're great.

Different teams use September call-ups in different ways. Rebuilding teams will often bring up young prospects and essentially give them a month-long audition in the big leagues. They get their feet wet and clubs can see where they fit in their plans for next season. Contending teams tend to stick with their regular players, at least until they clinch a postseason spot, so most of the time they use their September call-ups in blowouts or emergencies only.

As for postseason eligibility, there are three things to know about September call-ups:

  1. The player had to be in the organization by 11:59 p.m. ET on August 31 to be eligible for the postseason roster. No exceptions. They didn't have to be on the MLB roster or even the 40-man roster, but they had to be in the organization. That's why there's always a small rash of trades on August 31.
  2. Any player on the 40-man roster on September 1 is eligible for the postseason roster, even if they're still in the minors. Once upon a time only players on the 25-man active roster on September 1 were eligible for the postseason roster. That has changed. Anyone on the 40-man is eligible.
  3. Non-40-man roster players can be added to the postseason roster through a loophole. A player can be added to the 40-man in September and carried on the postseason roster as an injury replacement for a player who was on the 40-man roster. So if a regular gets hurt, it's possible to replace him on the postseason roster with someone who wasn't on the 40-man on September 1.

As you could imagine, MLB teams spent Friday calling up reinforcements now that rosters have expanded. If nothing else, they brought up some extra bullpen arms and maybe a third catcher. Not everyone who gets called up is a prospect. Some call-ups are veteran journeymen looking to hang around a little longer.

Here are five of this year's most notable September call-ups.

A's shortstop Franklin Barreto

Infielder Franklin Barreto spent some time with the Athletics earlier this season, hitting .190/.261/.381 with two home runs in 11 games. While in Triple-A, the 21-year-old authored a .290/.339/.456 batting line with 15 home runs in 111 games. MLB.com ranks Barreto, who came over from the Blue Jays in the Josh Donaldson trade, as the 37th best prospect in baseball. It's unclear if Barreto will remain at shortstop long-term, but, even if he has to move to second base, he'll have a spot in the A's lineup. He is an important part of their future and he figures to get a long look in September.

Cardinals pitcher Jack Flaherty

This week's Mike Leake trade opened up a rotation spot for 21-year-old Jack Flaherty, one of the most polished and impressive pitching prospects in baseball. He threw 148 2/3 innings with a 2.18 ERA and a stout 147/35 K/BB between Double-A and Triple-A this season, and the Cardinals figure to give him a regular rotation turn in September. Flaherty, who MLB.com ranks as the 53rd best prospect in baseball, is a super early 2018 NL Rookie of the Year candidate.

Cubs pitcher Dillon Maples

What a wonderful story Cubs right-hander Dillon Maples is. The 25-year-old nearly quit baseball last year after throwing only 95 total innings around injuries from 2014-16. He reached Double-A for the first time this season, and wound up throwing 63 1/3 innings with a 2.27 ERA and an even 100 strikeouts at three minor league levels. Maples spoke to MLB.com's Carrie Muskat about getting the call:

"(My father is) the guy I called last year when I wanted to hang it up," Maples said. "I'd lost passion, lost drive. I remembered calling him last year. It was appropriate that he would be the first one I called to tell, and my mom was in the car, and she found out right away. I heard her scream. It's definitely been a crazy ride for all of us." 

Pretty awesome. Maples has been through a lot of ups and downs in his career, but at long last, he is finally a big-leaguer.

Indians catcher Francisco Mejia

For my money, Indians catcher Francisco Mejia is the best prospect in the minors. Well, I guess he's not in the minors anymore. He was called up September 1. Mejia, 21, hit .297/.346/.490 with 14 home runs and a tidy 13.8 percent strikeout at Double-A this season. A switch-hitting catcher who can do that and play strong defense is one of the most valuable young assets in the game. MLB.com ranks Mejia as the 16th best prospect in baseball. I think that's too low, but no one asked me.

Dodgers outfielder Alex Verdugo

The Dodgers opted to call up outfielder prospect Alex Verdugo, not Joc Pederson, when rosters expanded Friday. The 21-year-old Verdugo had a big season in Triple-A, hitting .314/.389/.436 with 27 doubles and six homers in 117 games. He also had more walks (52) than strikeouts (50). If Pederson doesn't snap out of his ongoing slump, it's not out of the question that Verdugo could sneak onto the postseason roster as a reserve outfielder and pinch-runner.