One of the more overlooked assets that teams can acquire while wheeling and dealing is a trade exception. There are salary cap exceptions that teams can use to acquire players whose salaries are no greater than the amount of the exception plus $100,000.

So if a team is over the cap and wants to trade for a player without having to send back equal salary, no problem! As long as they have a trade exception large enough to swallow the incoming player's salary. The Boston Celtics used some of these exceptions already this season to facilitate mid-season acquisitions of key contributors such as Jae Crowder and Jonas Jerebko.

Trade exceptions expire after one year and can't be combined with other exceptions (including other exception types, such as the mid-level exception). The Celtics have over $25 million worth of trade exceptions, but that does not mean they can trade for a $25 million per year player without sending any salary out in return. In fact, most of Boston's trade exceptions are either partially used already or for such insignificant amounts that they likely won't ever be used.

Here's a breakdown of the trade exceptions that Boston currently has available, courtesy of HoopsRumors.com, categorized by their value and how conceivable it is that they could be used. This list indicates the amount available for each exception, how it was obtained, when it expires and how a portion was already spent for those that are partially used.

Insignificant

(These exceptions are partially used and therefore don't amount to enough to be realistically used)

Amount: $285,816

Obtained: Keith Bogans (Cavaliers)

Initial Amount: $5,285,816

Used: Brandan Wright ($5,000,000)

Expires: 9/25/15

Amount: $152,757

Obtained: Joel Anthony (Pistons)

Initial Amount: $3,800,000

Used: Jameer Nelson ($2,732,000), Jae Crowder ($915,243)

Expires: 10/17/15

Amount: $500,000

Obtained: Brandan Wright (Suns)

Initial Amount: $5,000,000

Used: Jonas Jerebko ($4,500,000)

Expires: 1/19/16

Low Value

(These exceptions could be used to acquire players on contracts close to the league minimum or prorated deals made mid-season)

Amount: $507,336

Obtained: Dwight Powell (Mavericks)

Expires: 12/18/15

Amount: $625,280

Obtained: Jameer Nelson (Nuggets)

Expires: 1/13/16

Amount: $689,840

Obtained: Austin Rivers (Clippers)

Initial Amount: $2,439,840

Used: Gigi Detome ($1,750,000)

Expires: 1/15/16

Amount: $1,334,092

Obtained: Kris Humphries (Wizards)

Initial Amount: $4,250,000

Used: Will Bynum ($2,915,908)

Expires: 7/19/15

Amount: $1,336,394

Obtained: Marcus Thornton (Suns)

Expires: 2/19/16

Enticing Value

(These exceptions could be used to potentially acquire a valuable veteran)

Amount: $7,707,865

Obtained: Tayshaun Prince (Pistons)

Expires: 2/19/16

Amount: $12,909,090

Obtained: Rajon Rondo (Mavericks)

Expires: 12/18/15

Summary

These options are a bit less appealing than the $25 million total we started with, but there are still some very useful assets here. Most of these trade exceptions will go to waste, while others could potentially be used to pick up pieces off the scrap heap mid-season.

The most interesting exceptions to keep an eye on are the ones created from the Rondo and Prince trades. The $12.9 million exception from the Rondo trade expires well before the deadline, so if the Celtics intend to use it then they should explore options to fill that exception this offseason. Once the season begins, there may not be many options available in mid-December that would help Boston upgrade its roster considering most teams won't have ruled themselves out of the hunt that early.

Boston has a bundle of future draft picks, including potentially four first-round draft picks in 2016. A team out of contention that is looking to shed some payroll (perhaps in an effort to avoid the luxury tax) may offer a veteran in exchange for one of those picks, while Boston could use one of those exceptions to take on their salary without having to surrender a player in return.

Trade exceptions go unused all the time, but Danny Ainge has done a masterful job of turning them into assets. He has made some trades that seem insignificant at first glance, but ended up creating a trade exception that was later put to good use. We saw how Boston was able to restructure their roster on the fly last season by acquiring role players that ended up making an impact.

The Celtics already have a lot of options available to them this summer as they look to add talent to their roster, but these trade exceptions serve as a way to open up even more options.