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.
On boats with high booms, this is not a practical location for the leech line cleat. A better solution is to upgrade to an “Over the Head Leech Line” where the adjustment for the leech line is at the tack of the mainsail. The leech line starts at the clew of the sail, runs up the leech, and then comes to a block at the head of the sail that turns it down the luff to the cleat just above the tack of the sail. This is a more expensive option, but a necessity on boats with high booms.