The No. 4 and No. 5 on a rugby union team are called the Locks. They are often referred to as the 'Engine Room' of the team as Locks not only add power to the forward effort but are also known for their work rate. Lock forwards pack down behind the props and hooker to make up the second row of the scrum and this is why they are also known as second row forwards.
Roles within the team
Tighthead Prop and a Lock more renowned for his lineout prowess. The latter would be the primary target in the lineout and would generally jump as the receiver in the middle of the lineout, while the former would jump at the front of the lineout. In general play, not many locks have become known for their ball-carrying ability as generally they have a high centre of gravity but instead are used to particularly good effect in mauls, as well, as tackling close to rucks.
Locks are usually very tall and thin as this makes them more effective and easier to lift in the lineouts, but some locks more known as good scrummagers and are bulky as well as being very tall.
Some of the best second row forwards in the world include Victor Matfield of South Africa, Paul O'Connell of Ireland, Richie Gray of Scotland and Joe Launchbury of England.