Beach cricket - tip and run, or 'one hand, one bounce'. Crazy golf - the distance from the side at which you can reposition your ball. Monopoly - landing on Free Parking and claiming the jackpot. Strip poker - do socks count individually, or as a pair?
House rules are usually introduced into games to make them easier or more entertaining, but the crucial point is that the rule must be agreed upon before the game begins. Failing to do so often results in furious disputes, bitter disagreements and general confusion - all of which were in evidence at Villa Park on Sunday.
The post-match kerfuffle in the Second City implies football could do with a few more pre-determined house rules. Even though an official set of laws was first ratified almost 125 years, and has been tweaked and added to constantly down the years by the International Football Association Board (IFAB), there remain significant shades of grey in the make-up of our beautiful game. For example, we often find ourselves debating the offside rule, and the active or passive roles of the attackers in question; whether you should kick the ball off the pitch when an opponent is injured, or play to the whistle; and whether a sliding tackle should never be deemed a foul if the defender has made contact with the ball.
Some would argue that these bones of contention, and others which crop up from time to time, enrich the sport. What else would Andy Gray and Richard Keys have to discuss at 6pm on a Sunday? What else would we all talk about down the pub? What else would keep the likes of Jeff Winter involved in football when he should have been forgotten about years ago? This seems all well and good. until it's your team on the receiving end of a monumental decision for which there is no definite right or wrong answer.
So to Sunday's derby between Aston Villa and Birmingham, and the 82nd-minute incident involving Blues centre-back Roger Johnson and Villa striker Gabriel Agbonlahor. As described in our match report, "Johnson looked like he got the ball when he challenged Agbonlahor in the box but referee Martin Atkinson decided he had brought down the Villa player and pointed to the spot."
Firstly, Johnson did indeed 'get the ball'. Agbonlahor was racing away from him but the defender stretched out his long legs and his boot touched the ball - not with any great force, but touched it nonetheless. As for how it looked to Atkinson, we don't know. The official had barely got out of the centre circle at the time the challenge was made. He considered what he had seen from a distance - two rival players with their backs to him going to ground - and decided to award a penalty to Villa. FIFA actively discourage referees from giving post-match interviews, and it was no surprise when Mr Atkinson later failed to appear to explain his actions. Mike Riley recently took over from Keith Hackett as full-time referees' chief with the Professional Game Match Officials (PGMO) board, but he has yet to give his view on the incident either - and will almost certainly avoid doing so.
Managers know this kind of tackle - one that takes the ball, but then takes the player - is a grey area. According to Law 12 of the rules, a tackle is adjudged to be an offence if it is "careless, reckless or uses excessive force". Do any of those descriptions apply to Johnson's tackle? If your response is yes, then a penalty was the correct decision. But remember - you've seen countless replays of the tackle, in slow motion, from different angles, and compared it to similar tackles you've seen from previous matches. Perhaps that is why Blues boss Alex McLeish's initial complaint was that, in his view, Atkinson could not have made a reliable assessment in a second or two from his position 35 yards away.
It's conceivable that UEFA president Michel Platini's Europa League experiment - extra officials watching the penalty area - would have helped here, but it's not guaranteed. And even if Atkinson was some sort of bionic referee, capable of keeping up with jet-heeled Agbonlahor and providing razor-sharp snap judgements, would he have made the 'right' decision? The toe of Johnson's boot moved the ball slightly further in front of Agbonlahor, but it didn't change its direction. Thereafter, the defender's momentum took him through the feet of Agbonlahor, and brought the striker down.
Writing in Monday's Daily Mail, former referee Graham Poll firmly backed Atkinson: "Martin Atkinson had a superb game and was correct to award a penalty to Aston Villa. There is a widely-held view that if a player touches the ball then it cannot be a foul - that is not correct.
"Roger Johnson did play the ball first but it remained within playing distance of Gabriel Agbonlahor. Johnson's challenge meant that he then brought the Villa striker down."
Many will echo Poll's opinion - that just because Johnson touched the ball, it does not mean he did not commit a foul. In last season's Champions League semi-final at the Emirates, there was a similar incident when Darren Fletcher stretched his leg across Cesc Fabregas in the penalty area to get a toe to the ball - felling the Spaniard in the process. On that occasion, Fletcher conceded a penalty and was also shown a red card. However, at Anfield earlier this season, Liverpool's Jamie Carragher tackled Manchester United's Michael Carrick just inside the penalty area. Carragher's boot made contact with the ball first but he then 'cleaned out' Carrick too - yet referee Andre Marriner did not then award a spot-kick.
Any of these tackles may not, in your opinion, have been careless, reckless or used excessive force, but they all did "impede the progress of an opponent" - something that, according to Law 12 and in the opinion of the referee, should result in the award of an indirect free-kick. "An indirect free-kick?! Well, that's a stupid idea," I hear you cry. But that's how it's described in the rulebook, so wouldn't that decision be fairer than pointing for a penalty?
Here's a house rule suggestion: if the defender's tackle connects with the ball before the player, that means he's timed it well and he shouldn't be penalised. As long as we've all agreed on it beforehand, there shouldn't be any squabbling afterwards. Let's send it in to IFAB. hopefully they'll get around to considering it sometime in the next 125 years.