Second rows are considered the engine room of rugby scrums; they are traditionally the hard men of most teams and very often make great leaders, as we see from our selection below.
They need to be big, powerful men, able to lead the physical confrontations and also to offer an option in the lineout.
We rank the six of the greatest locks in rugby history.
Willie John McBride is remembered more for his amazing achievements in the red of the British and Irish Lions than he is for the 63 caps he won for Ireland.
McBride, quite simply, is the face of the Lions, having toured a remarkable five times and having appeared in no fewer than 17 Tests.
The big Ulsterman is widely regarded as one of rugby's great leaders, having led the Lions pack on the victorious 1971 tour of New Zealand before taking the captain's armband for the trip to South Africa three years later, when the Lions went unbeaten.
The stories that surround these trips are legendary and very often involve the mischievous guile and wisdom of McBride.
He was the peacemaker when the Lions squad ran amok in hotels but, on the field, he refused to let his team take a backward step.
Check out this footage of his famous 99 call which showed the Springboks that the Lions were not going to be intimidated.
Colin Meads' international career with New Zealand spanned a remarkable 14 years, during which time he won 55 caps and played a major part in series wins over all the major Test nations, as well as the Lions.
A farmer from Te Kuiti on the North Island, Meads came to represent the typical hard All Black forward and, to this day, is an icon of New Zealand rugby.
Despite his "Pinetree" nickname, Meads was no giant, but his influence on the pitch was unquestionable.
After a year disrupted by injury, there were some who doubted the wisdom of selecting Paul O'Connell for the 2013 Lions tour of Australia.
How wrong they were—the 33-year-old wasted little time establishing himself as one of the Lions' key forwards with a remarkable workrate, lineout skills and leadership that had earned him the captaincy for the previous tour.
O'Connell's Lions days are probably behind him, but he still hopes to add to his 82 Ireland caps in what continues to be a remarkable career.
South Africa's Victor Matfield defined the modern second row: tall, powerful, mobile and with great hands.
During his 110-cap career with the Springboks, he formed a formidable second-row partnership with Bakkies Botha that played a crucial role in South Africa claiming the 2007 World Cup in France.
One moment from the 2007 final typifies what was great about Matfield, as he tracked back to prevent England centre Matthew Tait from scoring what could have been a match-turning try.
Needless to say, Matfield was named man of the match as the Boks registered their second World Cup triumph.
There is little that England icon Martin Johnson did not achieve in the game.
He captained England to the 2003 World Cup, the Lions to a series victory in South Africa in 1997 and his club Leicester to five Premiership titles and two Heineken Cups.
Renowned for his fearsome approach and intimidating glare, Johnson led more by action than words and was the heartbeat of all the teams he played for.
His contribution to England's winning cause in the dying seconds of overtime in the 2003 World Cup final typifies Johnson's game.
Nicknamed "Nobody" (as in "nobody's perfect") by his teammates, Australia legend John Eales tops this list of all-time great locks on account of being a double World Cup winner.
Oh, and the small matter that he was also an extremely-reliable goal-kicker who landed 31 conversions and 34 penalties during his 10-year Wallabies career.
As a 21-year-old, Eales packed down in the Australia boiler room during their 1991 World Cup triumph in England before captaining them to victory eight years later in the final in Cardiff.
In all, Eales won 86 caps for the Wallabies and brought the curtain down on a remarkable career after leading Australia's fightback in the 2001 series win over the Lions.
How the Wallabies could do with Eales right now.