The number six is the guy who cleans out the rucks and is primarily a defensive player on the blind-side, shutting down the opposition number eight or number nine.
He needs to be someone who likes that confrontation and physically robust approach, and of the three back-row positions he needs to be the absolute bedrock.
The role hasn't changed much over the years, a blind-side flanker still has to make sure that the opposition doesn't get over the advantage line - it's his job to smash them back.
If it has changed at all it's in the ball-carrying, not necessarily the off-load, but you have to be able to carry the ball to defensive lines and through defensive lines.
The All Blacks, for example, play a high-risk level of rugby these days, punching holes and then using short interplay to get in behind the opposition.
For this, the blindside and the rest of the back-row are often used more as three-quarters to punch holes because they're bigger and can draw two defensive players, and if you can do that you've created a hole somewhere else.
My idea of world-class number sixes would be New Zealand's Jerry Collins or the old All Blacks legend Michael Jones.