• Teams have to pass players through revocable waivers to trade them after the July 31st deadline.
• Players acquired after August 31st can't play in the postseason.
• Teams will often put most of their players on waivers to determine interest, since they don't have to give up every player who's claimed.
• Unclaimed players can be traded to any club in August (or even September).
• Teams don't have to trade players who are claimed. They can hand the player and his salary over for nothing.
• If a team places a player on waivers after he is claimed, but not traded, the team loses the right to pull its player back. In other words, the waivers are revocable at first, but not revocable afterwards.
• Clubs have two days (48.5 hours) to deal claimed players, but they can only negotiate a trade with one team. Teams have two days to complete a deal regardless of which day of the week the claim takes place on.
• If only one team claims a player, he can only be dealt to that team.
• If more than one team claims a player, he can only be traded to the claiming team in his league with the worst record.
• If a player's only claimed by teams in the other league, he can only be dealt to the claiming team with the worst record.
• Teams cannot pass players on the disabled list through waivers.

Thanks to Cot's Baseball Contracts and this article by ESPN.com's Jayson Stark. This post was first published on June 25th, 2009.

• Type A and Type B designations have been eliminated. Instead, teams will have to make players a qualifying offer to be eligible for draft pick compensation. 
• The qualifying offer, which will be determined by averaging the top 125 player salaries from the previous year, is expected to fall in the $12-13MM range for the coming offseason. All qualifying offers are for the same duration (one year) and the same amount ($12-13MM). 
• Teams will have until five days after the World Series to make qualifying offers and the players will have seven days to accept.
• Once a team makes a qualifying offer, the player has two choices: he can accept the one-year deal or decline in it search of other offers. If he declines the offer and signs elsewhere, his new team will have to surrender a top draft pick (the selection doesn't go to the player's former team). 
• Teams that sign free agents who turned down qualifying offers will surrender their first round picks. However, the forfeited picks don't go to other MLB teams. Instead, the first round simply becomes condensed.
• The first ten selections in the draft are protected. Teams with protected picks will surrender their second-highest selections. 
• The player’s former team will receive its compensatory selection at the end of the first round. Teams now obtain one compensatory selection, instead of two.
• If teams don’t make a qualifying offer, the player can sign uninhibited.
• Only players who have been with their clubs for the entire season will be eligible for compensation.

Baseball is a numbers game and we use lots of stats here at MLBTR. Some of them are easy to understand (Prince Fielder hit 38 homers in 2011) and some of them aren't as simple (he posted a -5.2 UZR/150 last year). So here's a guide to some stats you see here and elsewhere. It's not meant to be comprehensive; there are lots more useful stats than the ones that appear below, but these are some important ones:

• OBP – On-base percentage shows you the percentage of time a player reaches base. The league average now hovers around .325. The NL got on base at a .319 clip this past season and American Leaguers reached at a .322 clip. To compute OBP, add hits, walks, and hit-by-pitches as the times on base, and divide this total by the sum of the player's at-bats, walks, hit-by-pitches, and sacrifice flies.
• SLG – Slugging percentage measures a player's extra base power. To calculate SLG, divide a player's total bases by his at-bats. Power hitters like Fielder and Albert Pujols regularly slug over .500, but league averages were approximately .400 this past season.
• You'll often see us list a player's batting average, on-base percentage and slugging percentage (always in that order) like this: .280/.340/.450.
• UZR/150 – The defensive metric Ultimate Zone Rating estimates the runs a defender saves or costs his team. UZR/150 shows a player's impact per 150 games played. Check out this two–part explanation for more detail and keep in mind that it's best to look at multiple seasons when evaluating a player's defense with UZR/150.
• K/9 – The number of batters a pitcher strikes out per nine innings pitched. Pitchers struck out 7.1 batters per nine innings in 2011.
• BB/9 – The number of batters a pitcher walks per nine innings pitched. Pitchers walked 3.1 batters per nine innings in 2011.
• HR/9 – The number of home runs a pitcher allows per nine innings. Pitchers allowed 0.94 homers per nine innings in 2011.
• GB % – The percentage of batted balls that are ground balls. The Cardinals led MLB with a 47.7% ground ball rate in 2011, while the Diamondbacks had the lowest ground ball rate in the league: 41.9%.
• SIERA - Skill-Interactive Earned Run Average estimates ERA through walk rate, strikeout rate and ground ball rate, eliminating the effects of park, defense and luck, according to Baseball Prospectus. It's one example of a defense independent pitching stat (DIPS).

Check out Baseball-Reference, FanGraphs and Baseball Prospectus for lots more stats and check out our transactions glossary and the one at Cot's for explanations of transactions terms. MLBTR first published a version of this post on April 2nd, 2010.

Trading in September is a lot like trading in August. Players who have cleared waivers can be traded to any team, but players who have been pulled back from waivers can't be traded. However, to be eligible for postseason play "a player must be on the Active Roster, Disabled List, Bereavement List, or Suspended List of that club as of midnight EDT on August 31 of that year," as Keith Law explains in this ESPN article. Because of that rule, most deals occur before September. 

Trades do happen in September, since acquisitions can help teams reach the playoffs. For example, the Phillies famously traded for Sparky Lyle in September of 1980. Lyle was a major contributor for the '80 Phillies team, even though he wasn't able to play in the postseason en route to the team's first World Series title.

Lots of PTBNLs are determined in September, too. For example, the Angels traded Sean Rodriguez to the Rays to complete the Scott Kazmir trade two Septembers ago.

A version of this post was originally published in 2009.

• The deadline is typically 11:59pm on August 15th. It gets pushed back during years that August 15th falls on a weekend. 
• The deadline doesn't apply to college seniors and it doesn't apply to players drafted out of independent leagues or those who don't play college baseball again between two drafts.

Here's what happens to players who don't sign: 

• High schoolers who enroll in a four-year college program after being drafted aren't eligible again until after their junior year of college or their 21st birthday. For example, Mark Prior wasn't available between 1998, when the Yankees drafted him, and 2001, when the Cubs did.
• Drafted players who go on to attend junior college are again eligible after their freshman and sophomore years.  
• College juniors who don't sign are available in the next draft.  
• Players can't be drafted by the same team twice in a row unless they give permission.  

This post was originally published on June 10th, 2009. Thanks to Cot's Baseball Contracts.

To tender a player a contract is to offer a contract, but non-tenders refer to a specific kind of offer: offers of arbitration. Rules and precedent shape the kind of salary a player can expect through arbitration, so players under team control usually get raises through the process. 

For example, Joey Votto isn’t eligible for free agency yet, but he and agent Dan Lozano have some say in his future earnings. If the Reds offered Votto $750K in arbitration this offseason, Lozano and Votto could counter with a $4MM proposal and win. Arbitration can be expensive for teams, since a player’s salary depends in part on his previous earnings and comparable players.

Players generally earn $400K or so for their first few major league seasons, so they’re usually relatively cheap in their first arbitration seasons, but players entering their second, third or (for super twos) fourth arbitration seasons stand to make more money if they’re tendered an offer. 

If an arbitration eligible player hasn’t performed well, but projects to earn a considerable amount, his team will likely consider a non-tender. That means they have turned down the option to negotiate a contract with that player through arbitration, but it doesn’t mean the player’s going to sign elsewhere.

Jonny Gomes and Jack Cust both re-signed with their former teams after being non-tendered last winter. Both took paycuts, so the Reds and A’s saved money, but they risked losing the players to rival teams. (After a player is non-tendered he hits free agency and can sign anywhere.)

It’s complicated, but here’s what you need to know: teams non-tender players when they would rather risk losing the players to another team than go through the potentially expensive arbitration process.

This MLBTR Glossary entry relies on information from Cot's Baseball Contracts and the current collective bargaining agreement. It was originally published on August 4th, 2009.

• Players born outside of the U.S., Canada and Puerto Rico aren't eligible to be drafted; they're signed as free agents instead.
• The signing period begins July 2nd.
• The youngest eligible players for this year were born in late August and early September, 1994.
• This year's international free agents must turn 17 by September 1st 2011, or the end of their first pro season, whichever is later.
• Players don't make their pro debut until the calendar year after they're first eligible to be signed. 
• They can sign with any major league team.
• Players can sign after August 31st if they don't do so immediately.
• A number of players older than 16 will sign as well. 
• Check out this article by Baseball America's Ben Badler to see how the Reds signed Juan Duran before anyone – even his own agent - thought he was eligible.

Michael Ynoa set a record in 2008 when he signed for a $4.25MM bonus. Miguel Sano was the most-hyped prospect of 2009, but there has not been similar buzz surrounding any of this year's prospects. 

Thanks to Cot's Baseball Contracts, Kiley McDaniel and Ben Badler. This post was first published on June 17th, 2009.

Teams looking to deal players without first putting them on waivers must do so by 4pm EST on July 31st. Last year we saw Cliff Lee, Victor Martinez and Jake Peavy dealt at the deadline. This year Lee, Roy Oswalt and Corey Hart are some big names to watch.

There are two major reasons the 31st tends to be such a busy day. Teams have developed a good sense of their place in the pennant race. Sellers no longer have illusions about contending and buyers have a clear idea of their needs. Secondly, teams want to deal before August, when pulling off trades for top talent becomes much more complex.

This post was originally published June 25th, 2009.

• Selig recommends a limit to keep bonuses down.
• There are no direct consequences for a club that awards a big bonus.
• Teams aren't supposed to know what the recommended bonuses are for other slots.

Thanks to Murray Chass's recent article on the subject. This post was originally published June 9th, 2009.

• January – Players file for arbitration and exchange figures with teams. This process took place from January 5th-20th this year.
• February – Salary arbitration hearings take place.
• March – Clubs can renew the contracts of unsigned players who aren't yet eligible for arbitration.
• June – The first year player draft takes place in early June (it was June 9th this year).
• June 16th – Before June 16th, teams can't deal newly-signed free agents without their permission.
• July 2nd – The International Signing Period begins.
• July 31st – Teams have to put players on waivers to deal them after July 31st, which is why we typically see a flurry of trades before the July 31st deadline.
• August 15th – The deadline by which teams have to sign their first round draft picks or lose the rights to their selections. Some players, like Aaron Crow, are exceptions to the rule. Some years are exceptions, too; the 2009 deadline fell on August 17th because August 15th was a Saturday.
• August 31st – For a player to be eligible for his team's postseason roster, he has to be in the team's organization at the end of August. As a result, August 31st is the last day for teams to acquire players through waiver trades if they want to have them for the postseason.
• September 1st – Rosters expand from 25 to 40.
• October – For 15 days after the World Series ends, teams have exclusive rights to their own free agents. Players can talk with other teams, but only in general terms.
• November – The GM Meetings take place. This year, the GMs will meet from November 9th-11th in Chicago.
• December – Clubs have to offer arbitration to ranked free agents in early December to be eligible for compensation picks if they sign elsewhere. Players have to decide whether to accept their team's offer within a week.
• December – Clubs set their 40-man rosters and take part in the Rule 5 Draft. 
• December – The Winter Meetings take place in December and major trades and signings usually go down. This year, the meetings take place in Indianapolis from December 7th-10th.
• December – Teams must offer contracts to unsigned players by mid-December (this year, it's December 12th).

Thanks to Cot's Baseball Contracts.

All of the information on Major League deals comes from Keith Law's 2008 ESPN article.

Thanks to Jim Callis of Baseball America.

Teams use options to shuttle players on their 40-man roster to and from the minor leagues risk-free. Here are a few specifics to round out this MLBTR Glossary entry:

• Once a player is on his team's 40-man roster, his club has three separate seasons over the course of which it can promote and demote the player without exposing him to other teams.
• Minor leaguers on the 40-man roster are on an optional assignment.
• They can be promoted and demoted an unlimited number of times within an option year. 
• Once a player's three option years are up, he must clear waivers before going to the minors.  
• Options aren't used on players who spend less than 20 days in the minors.  
• Players with five years of service time can't be optioned to the minors without consent.  

Thanks to Cot's Baseball Contracts.

Players who violate the terms of their contract can be placed on the disqualified list, which prevents them from collecting service time and money. For example, the Nationals threatened to put Alfonso Soriano on the disqualified list when he refused to play the outfield in 2006, as this AP story shows.

Thanks to Cot's Baseball Contracts.