Lines of Distinction

Hockey will make you fall in love from the moment you walk into the rink. All of that speed. Rushes zooming up and down the ice. Non-stop skating, circling, passing and shooting between whistles. Deft handling of stick and puck. Physical play in the corners.
Not to forget a live game's sounds:
The whap-whap of skates, pucks slamming into the end boards or muffling into a goalie's leg pads, a cacophony of stick noises. It makes you want to come back.

An unsung pillar of hockey's exciting pace is its seamless substitution of players amid the game action. Hockey is the only major sport in which substitutions can be made as play continues. In every game, you will see a seemingly chaotic "on the fly" entry of players jumping over boards onto the ice while teammates return gassed, typically stepping through an bench door open for a few seconds.
The ideal "shift" for NHL players is 30 to 40 seconds, with forwards usually skating shorter in length while defensemen might on the ice a bit longer. Players go all out of these shifts, leading to lots of recovery breathing once a player is back on the ice.

In today's NHL, most teams staff four coaches behind the team bench to manage which players enter and exit ice and when. One coach monitors the defensive pairs (left and right defensemen) and another coach strategizes the interchange of lines (a center with left and right wings). A third coach is attentive to what lines the opponent is sending out to the ice (more about matchups in a minute).
Most nights NHL coaches dress four lines of three forwards each (12 total) plus three pairs of defensemen (six total). That's 18 "skaters" plus two goaltenders, the starter and the backup, adding up to 20 players in uniform. Some coaching staffs choose to dress a seventh defenseman to cover possible injury, choosing to piece together who plays on the fourth line.
Learning more about the forward lines can elevate a hockey fan's enjoyment of the sport many fold. Among the benefits:

Some basics: NHL coaches think of their forwards in two groups - the top six and bottom six.
The top six would be the first line and second line. These lines tend to be the most highly skilled in generating "quality scoring chances" (a key stat for hockey analytics experts) and inevitably include the team's top scorers. The top two lines usually get the most time on ice (TOI) because these players create more offensive chances plus log additional time on the man-advantage power plays.
General managers and their hockey operations staffers devote significant time to considering who will be in their top six each season. Some players are holdovers for several seasons, others are acquired in trades, free agent signings or draft choices or bottom-six who have matured.
Most NHL front office brain trusts tend to prioritize finding strong first- and second-line centers who have elite scoring and/or passing skills but also give full effort on defense, playing what Seattle GM Ron Francis likes to call "a 200-foot game" from one end of the rink to the other.
As for the wingers, the ability to shoot accurately and explosively is a big plus to earn a spot on the first two lines. Wings with sure shots and the knack for being in the right place on the ice at the right time can mean the difference between making the playoffs or going home for the summer in early April.
Ricky Olczyk, NHL Seattle's recently named assistant GM, says part of developing a top six is to project which players will make that cut in upcoming seasons.
"We might scout a player who is bottom six with another NHL team," says Olczyk, referring to pro scouting. "But we like the way he sees the ice, senses where the puck is going. We start to project that he could be a top-six player playing on the first or second line."

Coaches are responsible for deciding which players are together on the team's lines. Some coaches like to establish consistent trios while other coaches are more inclined to change up combos, even during a game. There are still other coaches who might juggle one winger on a line but keep a center and the other wing player together because of their on-ice chemistry, particularly noticeable in passing and playmaking.
The bottom six forwards tend to log fewer minutes, though many third lines get nearly the same minutes if they can be stoppers of the opponent's high-scoring line. The third line is sometimes referred to as the checking line for the skill set to shut down or at least slow down the other team's top line. The third line often gets near-equal ice time during the postseason when games are more physical and more tightly played
The fourth line tends to play about half the time of the first two lines, but the fourth line still spells top players about six to 10 minutes a game. In the playoffs, many coaches roll out all four lines in short rushes to keep all players fresh. The fourth line is often called the "energy line" because their shifts give other players a chance to rest, plus their play can give a team an unexpected boost if the fourth line scores a goal or even noticeably disrupts the opponent's top line.
No coaches or players deny the importance of the getting the best offensive players-the top six forwards-on the ice as much as possible during a game. Getting your best scoring players out on the ice 30 to 40 shifts per game is a norm. But today's speed game in the NHL demands that teams run out four lines more often than not to conserve energy and allow coaches to play the best matchups.
Coaches who can get the right matchups are almost always the coaches who get their teams in the playoffs.