The Celtics  enter the offseason with four players under contract for next season, and what looks like a pile of cap space. Looks can be deceiving, however. That theoretical space doesn’t account for contract options, qualifying offers, two first-round draft picks and the biggest charge of them all, cap holds.
With so much uncertainty surrounding them this summer, consider this a primer on what to expect when the league re-opens for business on July 11. Most of the answers rely on the great Larry Coon’s Salary Cap FAQ , a must-read document for understanding the cap. Player salaries come via our own reporting, and Sham Sports contract page .
What is the salary cap for next season?
It will be determined by the league’s audit that will take place from July 1-10. Last year’s cap was $58 million, and it is expected to be in that range for the upcoming season.
When can free agents sign?
July 11 after the audit is completed. Players with expiring contracts become free agents on July 1. Teams can begin negotiating with them on July 1, but they can’t sign until after the audit is completed on July 10 and the cap is set for the season.
What are the rules on restricted free agents?
Teams must make a qualifying offer by June 30 to keep a free agent restricted. This typically affects players coming off the fourth year of a rookie contract and the amount of the qualifying offer varies depending on where there were drafted. As a free agent with less than three years experience, Greg Stiemsma  is also a restricted free agent.
Restricted free agents can negotiate with other teams, but their former team has three days to match an offer sheet. The old rule under the CBA was seven days. In order to extend an offer sheet, a team must have the cap space — or an exception — to do so. Once they do, that space is effectively tied up for three days until a final decision is made.
How much cap space do the Celtics have?
As much as around $20 million and as little as none. On July 1, the Celtics will not technically have any cap space.
Who is under contract for next season?
Paul Pierce : $16.79 million
Rajon Rondo : $11 million
Avery Bradley : $1.63 million
JaJuan Johnson : 1.089 million
Total: $30.5 million
What about players with options?
Brandon Bass : $4.25 million (player option)
E’Twaun Moore : $762,000 (team option)
Sean Williams : $915,852 non-guaranteed
Total: $36.5 million (If Bass exercises his option, as expected, and becomes a free agent: $32.2 million)
Who are the unrestricted free agents?
Kevin Garnett 
Ray Allen 
Mickael Pietrus 
Keyon Dooling 
Marquis Daniels 
Sasha Pavlovic 
Ryan Hollins 
Jeff Green *
*Note: By rescinding their contract offer before the start of the season, Green became an unrestricted free agent, but the team retains his Bird rights.
Any restricted free agents?
Greg Stiemsma: $1.05 million qualifying offer
How much will the first round picks cost?
The Celtics have two first rounders in the draft, which will be held on June 28. Those picks count against the upcoming cap the moment they are drafted, regardless of when they actually sign their deals. The Celtics have the 21st and 22nd picks in the draft and each pick carries a salary slot. If they move up in the draft, they will inherit a higher salary slot.
21: $1.05 million
22: $1.04 million
Total: $38.5 million (including Bass)
OK, so that’s about $20 million in cap space, let’s go shopping, right?
Not so fast. We haven’t accounted for cap holds.
Prospective free agents are still bound to their former team’s cap by their Bird rights. The rule is designed to allow teams to go over the cap to retain their own free agents, but it’s also in place to prevent teams from loading up on free agents and then re-signing their own players.
In other words, you can’t have both.
How much are cap holds worth?
In terms of the cap, everything is an asset until it turns into a liability. The Mavericks were able to get Jason Kidd  from the Nets  by including Keith Van Horn in a sign-and-trade, even though Van Horn had stopped playing but had not officially retired. The new CBA closes that loophole, but the Celtics have had no need to get under the cap, so those holds sit until it’s time to get rid of them.
Here are some of the more important holds, or cap charges:
Kevin Garnett: $22.3 million
Ray Allen: $15 million
Jeff Green: $11.1 million
Nenad Krstic: $8.3 million
Brandon Bass: $8 million
Keyon Dooling: $2.9 million
Stephon Marbury: $1.44 million (seriously)
Total amount on the cap including salaries, options, draft picks and cap holds: $119 million
How do you get rid of cap holds?
There are three ways.
First, the Celtics could re-sign one of their players. Their salary then immediately affects their current cap and is removed as a hold. Second, the player could sign with another team and take their talents, and their cap charge, elsewhere. Third, the Celtics could renounce their Bird rights. If you renounce a player, the only way to bring them back is to sign them with existing cap space.
What about free agent exceptions?
Salary cap exceptions can only be used by teams who are OVER the cap. That’s why they are called exceptions. If the Celtics renounced all their free agents and created that $20 million in cap space, they could not use their exceptions. That $20 million in cap space is what they would have to construct a roster.
Non Taxpayer Mid-level: This is the old mid-level exception, but it is only available to teams that are over the cap but under the luxury tax and below the $4 million “apron.” The tax threshold last season was about $70 million, making the apron roughly $74 million. Simpler: You can’t use this exception if it takes your team salary over $74 million.
The mid-level is worth $5 million for up to four years with 4.5 percent raises. It can be split among multiple players. (Consult Larry Coon for a more detailed explanation of the exceptions and the apron ).
Taxpayer Mid-Level: This is available to teams who are over the luxury tax and above the apron and is worth $3.05 million. The Celtics used this exception to sign Chris Wilcox .
Bi-Annual Exception: Again, a team must be below the apron to use this exception and it’s worth $1.95 million. This is how they signed Marquis Daniels after the 2009 season.
Room Exception: This is the exception to the exceptions and is part of the new CBA. This is for teams who are under the cap and therefore forfeit their other cap exceptions and is worth $2.55 million.
This is kind of a buzzkill, isn’t it?
Pretty much, yup.
This also makes Kevin Garnett’s decision pretty important, right?
Garnett’s next move holds the key to the offseason. If they can bring Garnett back on a shorter deal for less money, it will set up the rest of the summer and the future. First, they would then add pieces around the existing core of Garnett-Pierce-Rondo-Bradley to remain competitive now. Second, they would try to keep the books clear for a later date when there are more attractive options available.
To fill out the roster, the Celtics would probably go over the cap so several free agents could be back including Green, Bass and Pietrus using their Bird rights. They could then possibly add a lower-priced veteran free agent to help complement the mix, using any extra cap space and/or a free-agent exception.
If Garnett goes elsewhere, or retires, then the Celtics will be looking at a full-on rebuilding project and that’s when that cap space would become so vital.