What is the job of a cricket umpire and when can they refer decisions to a third official?
Let's start with the basics. On the field of play, there are two umpires officiating a match.
One umpire stands behind the stumps at the bowler's end of the pitch, while the other umpire stands at square leg.
At international level there is also a third umpire on the sidelines and a match referee.
The umpire at the bowler's end makes decisions on lbw appeals, no balls, wides and leg byes.
The square leg umpire will judge stumpings and run outs.
At the end of each over, the umpires change position.
The third umpire uses TV replays to rule on run outs, stumpings, whether a ball has hit the ground before being caught or when it is unclear if the ball has crossed the boundary.
However, the third umpire can only make a decision if they have been asked to do so by the umpires out on the pitch.
Their involvement in the game has becoming increasingly influential, with fans and commentators alike calling for technology to be used for every contentious appeal.
There have been a couple of instances where the issue has been highlighted.
In the sixth one-day international between England and India in 2007, umpire Peter Hartley initially gave Paul Collingwood not out as he attempted a quick single.
But when Hartley jogged in to replace the bails, the slow-motion replay was shown on the big screen despite Hartley not asking for a referral to the third umpire.
When the replay showed that Collingwood was out, and the Indian team celebrated, Hartley then motioned for a referral to the third umpire.
So did Hartley make a mistake? Well yes in not referring straight away to the third umpire, but as MCC laws state, "an umpire may alter his decision, provided that such an alteration is made promptly. This apart, an umpire's decision, once made, is final."
Law 27 continues: "A prudent umpire will always take a moment or two to consider the facts before him. No shame is attached to an umpire changing his mind if, in the final analysis, the decision is a correct one."
In the end Hartley was correct but he will no doubt regret not calling for the third umpire as soon as there was any doubt about the run out.
Another instance was in the second Test between India and England in 2006.
England opener Andrew Strauss was correctly given out when umpire Simon Taufel referred a catch to third umpire Arani Jayaprakash in the second innings.
However, umpire Darrell Hair did not ask for Jayaprakash's assistance when he gave Kevin Pietersen out caught by Rahul Dravid at leg slip.
But television replays clearly showed Harbhajan Singh's delivery had hit Pietersen's wrist rather than his glove or bat.
If an umpire is unsure about a decision, then the batsman is always given the benefit of the doubt, but with more and more referrals to the third umpire that doubt can be checked upon.
Only an umpire can give a batsman out, but only after an appeal from the fielding side.
If a batsman stands their ground and no appeal has been made by the fielding side, they should not be given out by an umpire.
In 2008, the ICC trialled a referral system - which allowed players to challenge the on-field umpires and have their decisions referred to the third umpire - in a Test series between Sri Lanka and India.
The dismissed batsman or the fielding captain could appeal by making a "T" sign with both forearms at shoulder height - and each team were limited to three unsuccessful challenges per innings.
Sri Lanka batsman Tillakaratne Dilshan made history on 24 July when he became the first player to successfully appeal against an umpiring decision.
After on-field umpire Mark Benson adjudged him caught behind off Zaheer Khan, he appealed to third umpire Rudi Koertzen who ruled he was not out.
The referral system will be trialled again during several Test series in 2009.
The umpires indicate no balls, byes, leg byes, wides, boundaries and sixes to the scorers, who keep a running total of the runs scored.
The match referee rules on disciplinary matters.
If a player disagrees with the umpire about a decision and argue their case, the match referee can fine the player in question.
The players cannot appeal for the TV umpire to be called on.
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