So, a major sporting event starts this weekend. Weeks of big guys up front thumping seven shades of hell out of one another to enable their smaller, quicker brethren to make plays in the open field, advancing a non-spherical ball upfield to score points...
That's right, the Rugby World Cup's on its way and the USA are facing Ireland in the wee hours of Saturday night (your time). Here's my stab at a gridiron fan's guide to watching rugby union; since the games have common roots, I'll try to explain rugby union (as opposed to its redheaded stepsibling, rugby league) in gridiron terms.
How to follow a game of rugby
What may be the most confusing thing for a gridiron fan is that the action flows more in rugby: there are no downs, no constant subbing in/out of players and players don't get 10 seconds of action followed by 30 seconds of relative inactivity. Conceptually, the games are identical in that teams are trying to create and exploit mismatches to enable them to move the ball downfield. Passes have to be laterals and there's no blocking; it's possible to kick the ball upfield for your players to run under and challenge for, but they have to start from behind where the ball was kicked. The forwards (numbers 1-8) are the big guys who damage one another for the ball and the backs (9-15) and the faster, more skillful ones who do damage with the ball. Players have to switch between offence and defence depending on which side has possession of the ball; until the ball goes dead (out of bounds or the whistle is blown), players play on. Possession can ping-pong between sides and turnovers are both easier to effect and not so devastating when they happen (except, of course, when it costs you surefire points).
The single most useful concept I can bring to you is to compartmentalise what you're seeing into what we call phases. A phase of play is from one set piece or breakdown to the next. Every time you see a scrum, a lineout or the ball's being dug out beneath a pile of bodies, it's the start of a new phase. String a few phases together and defences start becoming more ragged (like a defense facing a no-huddle) - backline players get stuck in rucks and mauls, forwards filling in in the defence aren't sure who they're meant to be picking up or are exploited by faster and more agile backs running at them... sooner or later either a try will come, or the defence will be forced to kill the ball (illegally stop it from coming out of a ruck or maul, to prevent the attacking team from scoring a try). This will result in a penalty which may be kicked for 3 points, but that's better than giving up a 5-point try (with possible 2-point conversion on top).
Scrums and lineouts are known as set pieces and are ways of restarting the game when there's been a technical infringement (most commonly, when the ball is knocked forward when dropped) or the ball goes into touch (over the touchline, or sideline). In the former the two sets of forwards mash into one another (like the two lines on a short yardage run) and in the latter, the team who were not responsible for taking the ball out of bounds (unless kicked for distance from a penalty) gets to throw into both sets of forwards lined up. In both cases, obviously the side putting the ball into the set piece has the advantage. Scoring directly from the set piece is rare as defensive backlines are lined up in position, though it's also an opportunity for the attacking backline to attempt to use a training ground move (e.g. using the blindside wing as a strike runner on the openside; if he isn't picked up by the defence, it creates an overlap for them to exploit).
So, that's more or less the overview and you can pretty much watch a game knowing just that. I've included some more technical and in-depth detail below regarding the practicalities of the pitch and officiating, and a brief outline of how rugby union players' positions translate to their gridiron counterparts. If there's interest, I'll keep returning to this thread for the duration of the World Cup to answer any questions that might come up and to explain terminology and concepts that have you wondering.
THE RUGBY PITCH, OFFICIALS & other technical concepts
From one tryline (goal line) to the other, the other major landmarks are: the 22m line (think: the 20) in either half and the 10m lines on either side of the halfway line. There are also 5m markings from each touchline (sideline); these come into effect for scrums and lineouts, as they have to be set up at least 5m from the edge of the pitch.
You'll hear mention of the openside and blindside. These refer to the sides of the pitch relative to where the breakdown (scrum, ruck, maul) is, laterally. Unless it's slap bang in the centre line, there'll be one side of the pitch with more room (the openside) and one side of the pitch with less (the blindside). There may not be much in it.
There's a single referee and two touch judges (who indicate when the ball goes out of play and can alert the referee to foul play). At senior levels there is also a 4th match official (replay official) who currently can only advise the referee, if asked to, on whether the ball was grounded in the goal area for a try.
Games are 80 minutes, in 2 40-min halves, with a 10-minute halftime. Time is only stopped for injuries, or when the game is held up (typically, multiple scrum resets).
The equivalent of a touchdown is a try, which requires the ball to be grounded in the opposition's goal area, and is worth 5 points. After a try is scored the team can attempt a conversion (worth a further 2), from the same lateral point as where the ball was grounded. Obviously, a try that is scored between the posts is easier to convert than one that is squeezed into the corner.
A ball that is kicked between the posts other than after a try, whether off the ground (when the defending team is penalised) or as a dropgoal attempt (permissible in open play), is worth 3. There is no equivalent of a safety; if a player is tackled in his own goal area, grounding the ball there, the opposing team is awarded a scrum 5m (minumum distance) from his team's tryline.
Two teams of 15. There is no rolling substitution and players participate in both attack and defence with the ebb and flow of the game. Players in shirts 1 to 8 are collectively known as the forwards or the pack (think: linemen / in the box guys) and those in 9 to 15 are the backs (skill position players / defensive secondary). Substitutes are #16-22 and can comprise of any combination of forwards or backs though, obviously, teams will seek to cover as many positions as possible. Once a player has been replaced by a substitute, they're not able to return to the game unless it was for a blood injury (can be off for up to 10 minutes) or a prop needs to return to the game to ensure the team has 2 props (see below: positions)
The forwards are easiest thought of in terms of where they are in the scrum...
1-3: the front row
On the team who are putting the ball into the scrum (in the diagram, it'll be entering the tunnel from the left), the hooker's job is to - wait for it - hook the incoming ball with the heel of his boot, back towards his side. The props provide the grunt. Front rows are usually big and squat, though you can also have a smaller, more mobile hooker (what you lose in the tight game, you gain in the loose). Think: centers and guards/DTs.
4-5: second rows (aka. locks)
Second rows are the tallest players on the team and provide more drive to chanel through the props. They're usually the #2 and #4 jumpers in the lineout (the hooker throws in to a certain spot according to a prearranged signal, the other 7 forwards line up and the jumpers are usually at 2, 4 and 6). Think: tackles/DEs, sort of (they're taller, quicker, and more athletic than the front row).
6-8: back row (openside and blindside wing forwards/flankers, and the no.8)
These guys are expected both to mix it up in the tight game with the ugly brutes who comprise the front five and to have the speed, fitness and skill to get after and use the ball in the open field, in attack or defence. They have individual specialisations but, in general, they're big skill players who should be playmakers in attack and/or defence. One is usually the #6 (tail) jumper in the lineout. Think: TEs, LBs, even RBs and FBs, possibly larger SSs.
The backs (skill position/defensive secondary) can also be split into where they line up, as half-backs, the three-quarter line, and the fullback [NB. in that diagram, the openside - i.e. the majority of the space - is to the right, and that's where the backs have lined up].
9-10: half-backs (scrum-half and out-half or fly-half)
Equivalent to the QB when their team has the ball, the scrum-half is the (usually) little guy who nips in to get the ball from where his forwards have been scrapping for it and (if he doesn't kick it for territory/kick-and-chase or run it himself) passes it to the fly-/out-half, who decides whether to run it himself, pass it along the backline, or kick it (for position, since there are no forward passes in rugby, or a dropgoal attempt for points). #10s are most like punters and often the weakness in the defensive backline, but you can't function without a decent one who can keep his team going forward and scoring points when he can get them.
11-14: the three-quarter line (two centres in the middle, the wingers on either end)
The wingers are equivalent to WRs/CBs and ST gunners/kick returners, depending on what's happening. They need to be fast. Centres can be divided into inside and outside centres; typically the inside centre (#12, the one who lines up closer to the #10) is a more physical, direct runner and the outside centre (#13) is shiftier and has better ball skills, but these vary; they need to be sound in defence, ideally able to kick for territory if needed, and physical enough to handle collisions. Think: safeties, bigger possession receivers going over the middle, even TEs, HBs/FBs and LBs. Centres can be very diverse.
15: the full-back
The last line of defence, these guys function in much the same way as a single high safety and punt returner (in conjunction with the wingers, who should drop back to provide support) when the opposition has the ball (though there's no such thing as a fair catch, unless he's inside his own 22). On the attack, a good full-back will accelerate from deep to act as a strike runner - the advantage is that he can delay his entry to the line and thus pick the angle at which he takes the pass (lateral) and hits the hole. They also must be able to kick out of hand; if they're caught fielding a long kick and are isolated in the tackle, it can be a quick turnover ball and score for the opposition.