Foul play is defined as "anything a player does within the playing enclosure [the field of play and a space of not less than 5m around it] that is against the letter and spirit of the game.
"It includes obstruction, unfair play, repeated infringements, dangerous play and misconduct which is prejudicial to the game."
Examples of obstruction include charging or pushing a player while running for the ball, except the permissible shoulder-to-shoulder.
A player cannot get in front of a teammate carrying the ball, thereby preventing opponents from tackling him.
In these cases it is the the non-ball carrier who is penalised for obstruction.
The other way obstruction commonly occurs is when the ball carrier unintentionally runs into a teammate in front of him.
If the team gains no advantage from this, because there are no tacklers nearby, the referee allows play to continue.
All other cases of obstruction result in a penalty kick.
Unfair play includes intentionally offending and throwing the ball away or in-touch, all of which result in a penalty kick.
When the ball is thrown from the field, the kick is taken 15m infield.
A penalty try can be awarded if one would probably have been scored if not for the unfair play. Time wasting results in a free kick however.
Most yellow cards this season have probably been awarded for repeated infringements.
Such infringements are considered as a matter of fact, whether the player(s) intended them or not.
If the referee considers a player or players to be involved repeatedly, for example by using hands in the ruck to slow the opposition's chances of quick ball, he will issue a general team warning. Infringe again and that player would be cautioned and sent to the sin bin.
In law, if the same player then repeats the offence, or any other foul play law offence which results in another yellow card, he must then be sent off - as a second yellow card to the same player automatically becomes a red.
For lower grade games the referee is remitted to relax the standard, allowing for the poor knowledge of law or lesser skills of these players.
Referees must be both firm and consistent when dealing with dangerous play and misconduct during a game.
Obvious examples of dangerous play include punching, striking, kicking or tripping an opponent, as well as trampling or stamping on any part of an opponent's body when on the ground.
There are several forms of dangerous tackling, including early or late tackling.
A player must not be tackled above the line of the shoulders, even if the tackler's arms ride up around the neck or head after initial legal contact.
To be a legal tackle, the tackler must use one or both arms, and not with a straight or stiff arm action.
Nor can a player be tackled while in the air, usually in the action of catching the ball.
Except at a scrum, ruck or maul a player must not hold, push or obstruct an opponent not carrying the ball.
Dangerous charging has crept more into the game as players attempt to move opponents out of the way at the breakdown such as a tackle, ruck or maul.
In such situations, players must not charge into opponents without using the arms.
Unfortunately, players these days are often coached to carry out such actions in the mistaken belief they are allowed to "clear" the man out.
There is no problem if the player uses arms and stays on his feet but simply charging into a ruck or maul with shoulders only and off the feet is very dangerous, especially to a caught up and defenceless opponent. The so-called "tip tackle" is another recent unwelcome aspect of foul play.
Lifting a player off the ground, where the feet go above the horizontal line, and dropping or driving them into the ground so the player's head or shoulders hit the ground first, is also considered dangerous.
Late charging a player who has just kicked the ball results in a penalty kick at the spot or where the ball lands.
Retaliation is also considered misconduct.
Unfortunately, in many cases of foul play, the referee only catches the retaliation because his attention is perhaps focused around the ball when the initial incident occurs away from it.
Unless there are assistant referees to observe the whole incident, the player who retaliates is the one who ends up being penalised.
The moral of the story - play fairly and you have a good chance of seeing out the 80 minutes of the game.