A refresher on NFL free agency rules and terms

Here's a refresher on some NFL free agency terms and rules as a primer for the official start of the 2017 league year this week.

Negotiating period: We're in the middle of the three-day negotiating window, which began midnight Tuesday, in which teams can contact representatives of pending unrestricted free agents to work out a contract. However, deals cannot be made official until free agency actually begins. Teams are not allowed to host visits or directly contact pending UFAs in this span. So pending free agents without a certified agent, such as tackle Russell Okung, cannot legally talk with teams about a new deal until free agency begins.

Official start: Free agency officially begins 3 p.m. CT on Thursday to start the league year. Then all 2016 player contracts expire. Before this deadline, teams must exercise all 2017 options on players they'd like to retain for next season. Teams must also place tenders on their own restricted and exclusive rights free agents they'd like to keep. And the trading period for 2017 officially begins.

Franchise player: Teams may designate one franchise player (deadline was March 1) to be paid at least the average of the top five players at his position. A franchise player may sign an offer sheet with a different team, but the original team can match any offer. Should an offer not be matched, the team can pick up two first-round picks as compensation for losing a franchise player. The Vikings have only used the tag twice in 24 years, most recently in 2011 on linebacker Chad Greenway.

Unrestricted free agent: An unrestricted free agent is a veteran who has completed at least four accrued NFL seasons (accrued = at least six in-season weeks on an NFL roster) with an expiring contract. Once free agency begins, UFAs are free to sign with another team without compensation needed to his old team. UFAs are the key part of the NFL's veiled compensatory pick formula, which awards teams with mid-round draft picks if a club lost more valuable UFAs than it signed the previous season. Key UFAs for the Vikings this offseason include Adrian Peterson, Matt Kalil and Captain Munnerlyn.

Restricted free agent: A restricted free agent is a player with three accrued NFL seasons. If a long-term deal is not signed, their team can keep them via one of three predetermined one-year deals known as a "tender," which is not guaranteed. These tenders are classified as first round ($3.91M), second round ($2.746M) and original round ($1.797M). A team can use whichever tender it wants regardless of a player's draft position. RFAs then have through April 21 to negotiate with other teams and sign an offer sheet. For Adam Thielen, who received a second-round tender Wedneday, the Vikings would gain a second-round pick from a team that signed Thielen away. A team can match any contract offer before losing its own RFA.

Exclusive rights free agent: An exclusive rights free agent is a player with two or fewer accrued NFL seasons. If their team makes an offer, generally as a one-year league minimum deal, that player cannot negotiate with other teams and must sign the offer or sit out the season.