The summer of 2010 promises a free agency frenzy the likes of which we have never seen in the NBA. The Celtics will not be players for LeBron James, Dwyane Wade and Chris Bosh, but they do have moves to make.

The salary cap confounds everyone, but it’s the key to understanding the offseason. The first thing to note is that the cap has not been set for the 2010-11 season, but the NBA estimated that it would be $56.1 million, which is down from last season’s $57.7 million.

The Celtics are over the cap, but there are still a number of ways they can make moves. (Numbers and notes are from Sham Sports and Larry Coon’s Salary Cap FAQ’s page, two sites that should be bookmarked for serious NBA fans.)

COMMITMENTS: $64.4 million for six players

This includes Paul Pierce’s player option for $21.5 million and Rasheed Wallace’s $6.3 million salary for next season.

If Wallace retires, the Celtics are still obligated to play him the money left on his contract and incur the hit against their cap. They could work out a buyout that would free some money, but that would primarily be for luxury tax purposes, and the buyout number would also count against the cap.

If Pierce opts out and signs elsewhere, the Celtics still wouldn’t be under the cap because of what’s known as cap holds.

CAP HOLDS: $78.7 million (also includes Pierce’s option)

A cap hold is basically a device the NBA uses to close loopholes that teams exploited in the past to sign free agents from other teams and also retain their own. It is not an estimated salary or an obligation to pay anyone that sum.  

When a player hits free agency, their team retains the right to go over the cap and offer them more money than any other team. This is known as having a player’s Bird rights, an homage to Larry Bird and Red Auerbach, who argued that teams should be able to have the advantage when one of their players hits free agency.

The trade-off for having Bird rights is that each player is assigned a number, or hold, that counts against the cap until one of three things happens.

1.     They are re-signed and then their new salary is counted against the cap.

2.     They sign with another team.

3.     The team that holds their rights renounces them.

For example, Ray Allen’s cap hold is $19.7 million. That counts against the Celtics' cap space until one of the above three things happen. If they renounce his rights, they forfeit the right to sign him later unless they have cap space.

In other words, just because Allen is a free agent, that doesn’t mean that the Celtics can use his money to sign another player.

The Celtics have a number of players with holds on them, including P.J. Brown, Michael Olowokandi and Scot Pollard. There’s never been a reason to renounce those rights because they’ve been over the cap, so there they sit.

For now, the Celtics are way over the cap, and unless Allen and Pierce both leave, they won’t be under it without renouncing everyone left on the roster. Even then, they wouldn’t have enough money to make a maximum-level offer to a free agent.

They still have four methods for adding players.

THE MID-LEVEL EXCEPTION

The exact amount of the mid-level exception won’t be known until the cap is set, but it was $5.85 million last season and will go down a little if the projections holds.

The Celtics can use all of that money to sign free agents. They can give it to one player, as they did with Wallace last summer, or they can split it as they did in 2007 when they signed James Posey and Eddie House.

The Celtics do NOT have what is known as the bi-annual exception, which was used to sign Marquis Daniels. Just as it says, the bi-annual exception can be used every other year.

THE VETERAN MINIMUM

The Celtics were able to sign Shelden Williams for the minimum and well as add Michael Finley late in the season. You can go over the cap to sign players at minimum levels, which vary with experience.

Their pitch to veterans is: We’ll give you a chance to play for a championship.

SIGN AND TRADES

This is the big unknown this summer. Executives such as Darryl Morey with the Rockets have been saying that the sign-and-trade will be a huge factor in offseason moves.

Essentially it’s a way to facilitate player movement. With all their free agents, the Celtics could make great use of the sign-and-trade mechanism or they might not. The signs point to not at this juncture, but that could change.

THE DRAFT

The Celtics have two picks in Thursday’s draft, the 19th and 52nd.

In the first round they should be able to choose from a group of players that includes wing men and developing centers. Both would fill a need for the Celtics, who need to add some scoring punch and versatility to the bench and big bodies up front.

We’ll know a little more about their direction after the draft. Free agents can begin negotiating with teams on July 1 and officially sign contracts July 8.