Mike Celizic

The NBA’s salary cap is a joke. It’s been that way pretty much from the beginning, but it became official when the Dallas Mavericks and New Jersey Nets agreed to include Keith Van Horn in a trade that is essentially a swap of point guards — Devin Harris for Jason Kidd — and commissioner David Stern actually agreed to it.

Van Horn is retired, and has been for nearly two years. The Nets no more want him — they had him once, and it didn’t work out — than wolves want tossed salads. And Van Horn has no more desire to play basketball than Roger Clemens does to have Brian McNamee over for dinner.

Still, he’ll fly out to New Jersey on the Nets’ dime and spend the next month or so going to practice and doing nothing. In return, he’ll get more than $4 million and the Mavs will get Jason Kidd and the NBA can pretend its salary cap means something.

This isn’t the first time a player who doesn’t want to play has been traded to a town he doesn’t want to be in to make a lot of money for not doing anything so that the salary cap can remain the mighty force it is in bringing financial order to the NBA.

But because of the players involved, it’s the one that leaps out at us.

It’s also the one that inspired me to look up the salary cap rules just to see what we’re talking about. I tried to read them, I really did. But if they were a book, they’d be “Ulysses” and the IRS tax code would be “The Cat in the Hat,” only without all the two-syllable words.

In addition to being incomprehensible to anything less than six lawyers and four accountants, they’re also as useful as a pitch fork in a sand-shoveling contest. A salary cap by definition is a lid on salaries. Its purpose is to even the fiscal playing field and save teams — and the game — from economic ruin. Sounds simple, right?

Mavs trade for Kidd Hornets' Paul spoils Kidd's debut with Dallas Mavs finally complete deal for Kidd  |  Video Celizic: Deal proves salary cap is a joke Ventre: Kidd won't bring glory to Dallas Mavs, Kidd hope for better finish this time More coverage of the NBA

Not in the NBA, where trading a player who has no intention of actually playing is a perfectly legal maneuver.

To see how useful the NBA cap is, look at the team payrolls it’s supposed to control. The cap  this season is $55.6 million, which works out to more than $4.6 million for each of the 12 players on an NBA team.

The NHL and NFL have a “hard cap,” meaning teams can’t spend a nickel over a set figure. Baseball doesn’t have a cap, per se, but does charge teams a “luxury tax” for exceeding a certain payroll. It also takes some money from the rich teams and gives it to the poor teams via a revenue-sharing program.