The ball is oval, the games last 80 minutes and the posts are H-shaped, but anyone who has played both rugby union and rugby league will tell you that these things aside, they are two very different sports.

Wholesale variations in the laws and the numbers of players allowed per side (15 as opposed to 13) help to distinguish one from the other and, up until August, 1995, one was amateur (union) and the other professional (league).

Since rugby union went ‘open’, many players have been enticed to switch codes because of the sport’s greater global reach, or purely to try a different challenge.

While the intricacies of the breakdown and other nuances associated with rugby union have got the better of plenty of ‘cross-coders’, some have come out the other side and succeeded.

Measuring success is obviously a subjective exercise, and players like England trio Sam Burgess, Andy Farrell and Barrie-Jon Mather will argue they must have done something right to not only be capped but, in the first two instances, also play rugby union on the highest stage at a Rugby World Cup.

Selecting players with no previous top-level experience of rugby union, we have instead chosen three Rugby World Cup winners in Jason Robinson (2003), Brad Thorn (2011) and Sonny Bill Williams (2011 and 2015), while Lote Tuqiri was a beaten finalist with Australia in 2003.

Completing the super ‘set of six’ are prolific England winger Chris Ashton and Fijian phenomenon Semi Radradra.

Many quality players – particularly from Wales in the 1980s and early 90s – who went ‘north’ from rugby union to rugby league and then came back again, such as Jonathan Davies, haven’t been considered because they’d been capped in the 15-man code before making the switch.

Jason Robinson

Joining Sale Sharks in November 2000, Robinson quickly adapted to the different demands of rugby union and was capped by England barely four months later. Known as ‘Billy Whizz’ for his electric pace and dazzling footwork, the former Wigan and Great Britain RL ace terrified the life out of rugby union defences in an international career that spanned six years (2001-07) and included two Rugby World Cup finals. He scored England’s only try in the 2003 success against Australia and went on two British & Irish Lions tours.

Lote Tuqiri

Switched to rugby union relatively young, at the tender age of 22, having already played for both Fiji and Australia in international rugby league. The powerfully-built winger scored three tries in Australia’s home Rugby World Cup in 2003, including one in the final defeat to England, and finished with 30 tries in 67 tests. In 2008, Tuqiri surpassed Matt Burke's Super Rugby try-scoring record. He edges out RWC 2003 final team-mates, Mat Rogers and Wendell Sailor, on length of service.

Sonny Bill Williams

The charismatic centre has flip-flopped between the two codes at regular intervals, as well as competing as a heavyweight boxer. Following a highly successful stint at Canterbury Bulldogs, Williams crossed codes for the first time in 2008 to join Toulon, where his offloading ability revolutionised midfield play. However, his union career will be best remembered for his displays for the All Blacks, with whom he joined a select band of players to have won the Rugby World Cup twice. An all-round sporting prodigy.

Brad Thorn

One of a rare breed of cross-coders to succeed as a forward, the no-holds-barred second-rower holds the distinction of being capped by Australia in rugby league and New Zealand in rugby union. After making his name with Brisbane Broncos in rugby league, Thorn enjoyed a highly decorated career in rugby union, winning the Rugby World Cup (in 2011) as well as Tri-Nations, Super Rugby and Heineken Cup titles. In February 2015, he became the first player in the history of Premiership rugby to play as a 40-year-old, during his two-year stint with Leicester Tigers.

Chris Ashton

Came across from Wigan in 2007 and promptly scored with his first touch of the ball on his Northampton debut against London Welsh. Ashton has been a regular across the whitewash ever since with his uncanny ability to be in the right place at the right time, while his ‘Ash-splash’ trademark celebration has added a theatrical flourish to proceedings. In addition to scoring 20 tries in 44 tests for England, Ashton holds the all-time Heineken Cup and the Top 14’s single-season try-scoring records.

Semi Radradra

One of the standout performers at Rugby World Cup 2019. At 1.90m and 110kg Radradra is the size of a back-row forward, yet he plays centre, wing or full-back. Can bump tacklers off or to blast through them with his sheer physical presence, or alternatively, beat them on the outside with his deceptive pace. When not scoring tries himself, the Fijian’s offloading ability opens up holes for others to exploit. Against Georgia at RWC 2019, for example, he bagged a brace but set up three more tries for his team-mates. Initially made his mark in rugby league, with Parramatta Eels between 2013 and 2017.