The game is ever-changing, and the role of the fullback is becoming increasingly demanding.
Seldom is a number three able to sit back and defend for 90 minutes, with some now spending as much time in the oppositions half as their own.
Everyone loves to see attacking, free-flowing football nowadays, and the role of the wing back or full back is an integral part of allowing that to happen.
You can read RealSport’s best number fours feature HERE.
Defence to attack and back again… it’s part-and-parcel of the role, and over the course of a game, full backs in some sides may well cover more or just as much ground as a central midfielder.
Here are five players who can make a massive difference at both ends of the pitch.
1 Marcelo [Brazil]
Although typically donning the number six shirt for his country, Marcelo's name is almost synonymous with the position.
The 30-year-old Brazilian has played 52 times for his country, first appearing in 2006 before making the move to Real Madrid.
Marcelo has made 320 appearances for the Spanish giants, and his playing style has made him one of - if not the - most recognised left backs in the world.
INTERESTING FACT: Marcelo own goal at the 2014 World Cup was the opening goal of the tournament, as well as Brazil's first-ever at the World Cup.
Both his domestic and national side play football in a way that allows Marcelo to be forward-thinking and more aggressive than others, meaning he never really has to adapt his personal game.
Marcelo will feature in a Brazil team that already been tipped as one of the favourites to lift the trophy and boasting one of the more frightening attacks in the competition.
Should Brazil be allowed to play the expressive, fluid football that fans crave to see, Marcelo will lick his lips at the chance to do what he does best; get forward.
2 Benjamin Mendy [France]
Having spent most of Manchester City's title-winning season on the sidelines, Benjamin Mendy has developed more of a reputation for his social media presence than his on-field talents.
A season-ruining knee injury at the beginning of the campaign severely limited Mendy's chances to make an impact in Manchester following his high-profile move from AS Monaco at the end of last term.
Despite the stuttering start, Mendy is keen to get back to business on the pitch.
INTERESTING FACT: Benjamin Mendy has more international caps (7) than he has Premier League appearances for Manchester City (6).
Having featured heavily in France's pre-World Cup friendlies and warm-up games, he looks in pole position to start in Didier Deschamps' left back slot.
France, like Brazil, will showcase an array of fast-paced attacking talent, and should Les Bleus play the slick, dynamic football we all know they are capable of, then this could well play into Mendy's hands.
Pep Guardiola revolutionised City's style and identity this season, and although Mendy's involvement in that was short-lived, he'll be looking to make a mark in Russia this summer to give him the platform to go into the 2018/19 season on a high.
3 Jonas Hector [Germany]
Jonas Hector has had to wait his turn to represent his country.
After making his first appearance in 2014 as a 24-year-old, Hector is now on course to take the starting left back role at his first World Cup.
The stranglehold that now-retired Philipp Lahm had on a position in the German defence made it difficult for other players to break through and into the team.
After his retirement in 2014, though, manager Joachim Low looked to fresher faces to fill the gap. Hector has now made 38 appearances for Germany, including a solid run at Euro 2016.
KEY STAT: Jonas Hector scored the winning penalty against Italy to take Germany to the Euro 2016 semi-final.
Hector plays his league football for Koln in his native Germany though his side has now been relegated to the second tier after finishing a huge 14 points adrift of safety.
His standout performances for his club have raised his profile on the continent, and Premier League clubs Chelsea, and more recently Liverpool, have both expressed their interest.
Despite the scenario of playing in a lower division, which may hurt his chances of future national selection, Hector is committed to Koln and recently signed a new contract.
4 Jordi Alba [Spain]
Barcelona star Jordi Alba is the obvious choice at left back for a Spain side keen to get back to the top of the world game.
Alba has been a mainstay in the Spanish backline for the last few years, and his position looks safe for the tournament in Russia. Having amassed 62 caps for his nation, Alba will lineup alongside other familiar faces in Sergio Ramos and Gerard Pique.
KEY STAT: Jordi Alba (8) made the most assists of any defender in the 2017/18 La Liga.
Having played a part in a Barcelona team that is made famous for its possession-based, attacking football, Alba is used to having the licence to get forward and support attacks down the left flank.
His attacking talents and capabilities have seen him also used as a left-sided midfielder, though is more comfortable in his natural position.
Alba's stamina and work ethic have helped earn him a reputation as one of the world's more recognisable left backs - often attacking as much as defending, Alba's style of play requires high levels of fitness.
5 Nicolas Tagliafico [Argentina]
Of all the names on the list, Tagliafico's is probably the more unfamiliar.
Not since the days of Inter Milan legend, Javier Zanetti, have Argentinian fans had a staple feature in their left back position.
Tagliafico, 25, made the move to Eredivisie side Ajax at the end of the January transfer window, and has been a regular in the Netherlands ever since, making 15 appearances.
KEY STAT: Nicolas Tagliafico made two assists and scored once in 15 games for Ajax.
His pace, stamina and desire to always improve has seen him often likened to the aforementioned Zanetti; big things are expected of the Buenos Aires native.
Despite having only made five senior international appearances, Tagliafico looks set to be handed the starting berth in Russia.
His energy and unrelenting stamina will be welcome in an Argentina side that is always liable to concede goals and look frail defensively.
Listen to the RealSport football writers discuss Group D in Kremlins in the Basement: RealSport’s daily World Cup podcast.
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