Tradition has long maintained that the man should keep to the outside of the pavement.
When a couple change the direction of their walk he moves behind his companion to maintain his outside position.
The woman automatically moves to the right or left in response to his change of position.
More than two people walking
One man and two women walking - he continues to take the outside position; he does not walk between them.
One woman and two men walking - she does take the middle position.