However, some players have switched between the codes and Last Word on Rugby has decided to look at five of the best players to play both.
Sonny Bill Williams
Sonny Bill was born in Auckland but instead of representing the one NRL side in his home country, the New Zealand Warriors, he started out at the Canterbury Bulldogs after being spotted by a club scout.
At just 19 he made his debut in both domestic and international competition, becoming the youngest-ever test player for the New Zealand Kiwis.
His first NRL season ended in success as the Bulldogs won the 2004 Grand Final, with Williams becoming the youngest player to play in the showpiece event. It seemed that he would be a leading figure in the NRL until in 2008, Williams swapped codes controversially mid-season.
Citing salary cap issues as he moved to rugby union side Toulon, just 18 months into a new five-year contract with the Bulldogs. He played 33 matches for the French side, earning a runners-up medal in the 2009/10 Challenge Cup.
In 2010 he made his All Blacks debut despite having never played Super Rugby, the second person to play for the national side after first representing them in rugby league.
The following year saw him win the 2011 Rugby World Cup with New Zealand. Then in 2012 he finally won the Super Rugby title, the fourth person to win Grand Finals in both codes.
He returned to league with the Sydney Roosters, winning their player of the year in a season where they won the minor Premiership and NRL Grand Final.
Another move to rugby union saw him play in another World Cup as New Zealand finished third before his move to current team Toronto Wolfpack in league.
Hailing from Fiji, Radradra played for his country’s Sevens side, known as one of the best in the world, and was scouted by NRL team Parramatta Eels.
From his debut in 2013, the winger scored 74 tries in 92 matches, winning the Dally M medal for the league’s best winger in 2014.
He repeated this feat in 2015 when he finished as the league’s top-try scorer with 24 tries including breaking a 61-year record of scoring two or more tries in five successive games.
In 2018 he moved to Toulon where he played just 23 games in his one year spell but helped Chris Ashton break the Top 14 try-scoring record before he moved to Bordeaux-Begles, notching 35 tries in just 25 matches.
For the 2019-20 season finale he will play for ambitious English Premiership side Bristol Bears.
Despite being born in Leeds, Robinson made his Super League bow with rivals Wigan Warriors in 1991.
He went on to claim two Challenge Cups including a 1997 triumph over his hometown club earning the Lance Todd man of the match award, scoring two tries.
In 1994 he was part of the Wigan team who won the World Club Challenge beating Brisbane Broncos and the following year he played as England lost the World Cup final to Australia.
He played in two Grand Finals, scoring the only try against the Rhinos in the inaugural showpiece in 1998 and was a runner-up in 2000 as the Warriors lost to St Helens.
Soon he signed for rugby union side Sale Sharks days and in 2006 became the first person to win both the Guinness Premiership and Super League titles in each sport.
However, he is best known for his try in the 2003 World Cup final as England claimed their only World Cup, beating Australia in extra-time, one of 30 tries in 51 matches.
His debut in 2001 meant he was only the second man ever to play rugby union for England after having first played rugby league for Great Britain.
Welshman started at local side Neath where he played for five years, during which he made his Wales debut after just 35 club matches and won man of the match against England after a try and a drop goal.
He moved to Llanelli for a year in 1988 and soon after helped Wales win the Triple Crown, winning 37 caps, including four as captain.
One of the first players to do so, Davies crossed codes to sign for Widnes Vikings in 1989 , where he made his name in the code, making 126 appearances.
After his move for a world record £230,000, he won a World Club Challenge and the Man of Steel in 1993.
He played in the NRL for the Canterbury Bulldogs for the 1991 summer, scoring 100 points in his 14 games.
His time playing for Great Britain is best remembered for a stunning 50-metre solo try in 1994 against Australia at the old Wembley.
The New Zealander is a well-known rugby icon in his native country but he actually
started his career in rugby league with Australian side, the Brisbane Broncos. During the first of two spells, between 1994-2000, he won a Super League title, two NRL
premiership, and a World Club Challenge.
Playing as a second-row, he gained representative honours playing 14 matches for Queensland, as he won two State of Origin series.
Representing the Kangaroos in 1997, it was the pinnacle for any league player and Thorn was dominant in his role with the World Champions.
His switch to rugby union came in 2001 where he played the majority of his career at Crusaders and Canterbury, in two periods. The first seeing him earn several NPC and Super Rugby titles, and after first not accepting a place in the All Blacks, he participated in the 2003 Rugby World Cup.
In representing New Zealand in rugby, Thorn became the second man in history (after Bill Hardcastle) to rep for Australia in 1997/98.
Unusually, his second spell saw him return to league where fantastically Brad Thorn won another NRL Premiership in 2006, as he racked up 200 games.
At this point, switching back to Rugby Union was brought on by the fact he wished to help the All Blacks to win a World Cup. In 2008, he began that journey with the Crusaders. Achieving another Super Rugby championship, this saw Thorn play again for the All Blacks.
The league prop was a big player in the international arena, where his 59 caps at lock for the All Blacks included a triumphant 2011 Rugby World Cup campaign.
A move to Leinster beckoned in 2012 and he won another major trophy, the Heineken
Cup making him the first player a World Cup, a Super Rugby title and the Heineken
Cup. He has since been joined in this feat by Bakkies Botha, Danie Rossouw, and Bryan
Retiring from the Leicester Tigers, his last game was at the senior age of 40. 22 seasons as a professional has written Brad Thorn’s name in the highest echelon of the oval ball code.
These five players have made a mark in both codes of the game and in the coming years, there is sure to be other players to emulate them.
Other names have since been suggested; Sam Burgess, Inga Tuigamal, Frano Botica, Matt Rogers, Michael O’Connor, Mathew Ridge, Lote Tuquiri, Wendell Sailor, and Israel Folau.
“Main photo credit”