the sails on a tall ship's mast must all be turned together, because of all the gear that runs between them. the rate of turn is set by the course, the heaviest yard and hence the most difficult to move. the teams on the other braces for that mast must watch the course and keep their own yard in line with it. the braces may be marked with leather tags or twine seizings to indicate the centre ("square") position and the two extremes, though these marks may not always be accurate due to stretch in the line.