Rugby union is one of the few ball games where the ball cannot be passed forwards.
That means a player moving towards the opposition's dead ball line must pass the ball to a team-mate either along or behind an imaginary line running at right angles to the side of the pitch.
The same principle applies even when players are not passing the ball.
If they fail to catch or pick up the ball cleanly and it travels forward off a hand or arm and hits the ground or another player, it is called a knock-on.
The same applies if a player is tackled and the ball goes forward.
'Forward' means towards the opposing team's dead ball line
If a player fumbles the ball but catches it before it has hit the ground or another player, it is not a knock-on.
When a knock-on occurs, the referee will stop play and award a scrum to the team which has not knocked on.
If the ball is thrown forward at a line-out, a scrum is awarded 15 metres in from the touchline.
If the referee decides a player has intentionally knocked on or thrown the ball forward, a penalty is awarded to the other team.
And if the referee decides the other team would have scored a try if the intentional knock-on had not taken place, a penalty try is awarded.
The one exception to the knock-on rule is the charge-down.
If a player charges down the ball as an opponent kicks it, it is not a knock-on, even if the ball travels forward.