What is the Olympic curling double touching controversy about?

A controversy over allegations of cheating in curling at the Winter Olympics has made headlines all over the world and plunged the sometimes obscure sport into the mainstream.

Here's what you need to know about double touching:

What is double touching?

In a legal delivery, a curler must release the stone before the hog line, which lies 21 feet (6.4 m) from the centre of the tee line the curler pushes off from.

A curler can touch the handle of the stone as many times as they like before the hog line, but they cannot touch the granite portion of the stone while it is moving forward.

Doing so results in the stone being 'burned', or removed from play.

World Curling has introduced electronic handles at these Games, which flash green for legal throws and red when players release the stone past the hog line.

Each stone's handle is fitted with a touch sensor, which interacts with a magnetic strip embedded in the ice. The granite, however, does not contain any sensors.

How does officiating work in curling?

Curling has always prided itself on being an honour-based sport, with players calling their own fouls. World Curling does not use video replays to aid in officiating.

Earlier this month, World Curling board member David Sik, who chairs the governing body's technical commission, told Reuters umpires are rarely called into action.

"There is always an agreement between the skips and teams on how to play the game and how to behave," he added.

When did the controversy begin?

In a round-robin game at the men's competition on (13 February), Olympic champions Sweden accused Canada of cheating by double touching stones, after which there was a heated exchange between Marc Kennedy and Oskar Eriksson.

Sweden's Eriksson told Kennedy he would show him video proof of him committing a violation while releasing the stone. The Canadian twice responded with an expletive known colloquially as the F-Bomb.

Canada went on to win 8-6. World Curling said umpires had not noticed any violations while monitoring deliveries for three ends during the game.

Swedish media later released videos and images of 2010 Olympic champion Kennedy touching a stone with his finger while it was crossing the hog line.

The video, shot by Swedish broadcaster SVT, raised eyebrows as it was filmed from an angle not usually used in curling broadcasts, leading to Canada coach Paul Webster alleging that the incident was "premeditated and planned".

SVT said it had captured the footage by moving a camera into the stands to film the hog line after Sweden had complained about a violation early in the game.

Kennedy later said he regretted his angry outburst but would not apologise for defending his integrity after being accused of cheating.

The following day, World Curling said it would have two officials monitoring for rule violations during the rest of the competition at the Olympics.

It also reiterated that touching the granite was forbidden.

How did World Curling's decision affect the competition?

It is rare for rocks to be pulled for double touching during competitions, but after the officials were set on the hog line, Canada women's skip Rachel Homan and British men's curler Bobby Lammie both had stones removed due to violations.

Homan said the officials' decision to remove her stone during a defeat by Switzerland was "insane", while her men's team counterpart Brad Jacobs said it was "despicable" and that officials were not trained for the situation.

Following a meeting with representatives of participating National Olympic Committees, the governing body then walked back on its decision to increase officiating on Sunday.

They returned to a system similar to the previous method of monitoring violations, in which umpires would be set at the hog line to observe deliveries for three ends if there was a complaint from either team.

How have Canada and Sweden fared in the Olympics?

Canada and Sweden are the two most successful nations in the curling competition at the Winter Olympics, with 12 medals apiece. Canada have one more gold and lead the overall medal tally.

Canada is the curling capital of the world but the country has not won a gold medal in the men's or women's events since the 2014 Games.

At these Games, the Swedes are out of contention for medals after a poor campaign in the men's event. Canada and Sweden are still in medal contention on the women's side.

The brother-sister duo of Rasmus and Isabella Wranaa have already secured a gold medal for Sweden in Cortina, winning the mixed doubles competition last week.