Full-fledge NFL free agency has been in existence for 20 years. However, elements of free agency surfaced in the league well before Reggie White made his way to the Green Bay Packers in 1993.

Here's a look back at some of the strange twists of free agency before and after '93:

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1. The poison pill – Technically, it's still not illegal, but the NFL quietly discourages clauses in restrictive free agents' contracts that make it nearly impossible for the current team to retain the player. It has resulted in some absurd moments. In 2006, Steve Hutchinson left Seattle for Minnesota, the Vikings getting him for a seven-year, $49 million contract that included a poison pill that the Seahawks unsuccessfully challenged. In return, Seattle then signed Minnesota wide receiver Nate Burleson later in March 2006 to a matching seven-year, $49 million deal that included two poison pills. Here is the description of the two poison pills from then-ESPN reporter Len Pasquarelli: "The first would guarantee the entire contract, all $49 million, if Burleson plays five or more games in the state of Minnesota in any season of the contract. The Vikings, of course, play home games in Minneapolis, at the Metrodome there. The second bizarre provision would guarantee the full contract if Burleson is paid more on average per year than all of the Minnesota running backs combined. At least for now, the averages of the Vikings' tailbacks fall well shy of the $7 million average of the Burleson offer sheet."

2. Early courting? – While there wasn't supposed to be contact between players and new teams before the start of free agency in past years (this year features the first legal tampering period), there have been some questionable moments. In 1994, safety Gene Atkins just happened to check into a hotel less than a quarter-mile away from the Dolphins training facility on the night before free agency opened. Atkins signed on the first day. Three years ago, Detroit coach Jim Schwartz showed up at the front door of defensive end Kyle Vanden Bosch's house just after midnight after free agency opened.

3. Plan B free agency – From 1989 to 1991, the NFL came up with a bizarre form of free agency in an attempt to soothe players. It allowed teams to protect 37 players and allow others to be free agents. The result actually angered players more because the few decent players who were free ended up getting paid better than some protected players. A group of 8 players eventually sued and won the case in 1992, opening the door for free agency in its current form.

[Related: Anquan Boldin refuses pay cut, released by Ravens]

4. John Riggins – In 1976, a temporary opening in the collective bargaining agreement allowed Riggins and 17 other players to change teams. Riggins, a former first-round draft pick and a noted free spirit with the Jets, signed a four-year, $1.5 million contract with Washington. The running back sat out the entire 1980 season following a contract dispute, then helped the Redskins capture a Super Bowl title two seasons later.

5. Wilbur Marshall – In 1988, Marshall forced his way out of Chicago, where he had been part of the Bears' great 1985 championship team and defense. The Redskins, making Marshall the first free agent in 11 years to switch teams, paid a steep price when the Bears declined to match a five-year, $6 million contract offer. Washington had to give up two first-round picks for the linebacker, who helped the Redskins win another Super Bowl in 1991.

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