All UK Sailmakers' sails come standard with a leech line that runs under the back edge of the mainsail. It can be tensioned to prevent the leech from fluttering. The standard arrangement is to have the line run from the top of the sail down to a cleat just above the clew. 

The standard location for a leech line cleat is just above the clew of a mainsail.

Mainsail options: over the head leech line.