Two solid lines traversing the width of an NHL rink that divide the ice surface into the attacking, defending and neutral zones. The lines are blue in color and each measure 85 feet (25.91 meters). The space between a team's goal line and the blue line closest to it is the defending zone, while the space between the other blue line and the opponent's goal line is the attacking zone (these zones are 64 feet or 19.51 meters long). The space between both blue lines is the neutral zone (the neutral zone is 50 feet or 15.24 meters long).
The blue line is critical to the application of the offside rule. No player can cross the blue line and enter the attacking zone before the puck; if he does, an offside is called on that player. Besides the two referees overseeing the game, there are also two linesmen present on the ice surface during an NHL game, and their main responsibility is to make sure that players wait for the puck to cross the blue line before they enter the attacking zone.
When a team settles into the attacking zone during an even strength situation, defenders playing on that team will position themselves near the blue line and will support the attack if needed. During a power play situation, one or two players on the attacking team will position themselves at the blue line, either to pass the puck to teammates and set them up with scoring chances or to take a shot (usually a one-timer) when receiving a pass. Players positioned at the blue line on the power play are said to be "playing at the point".