Yawl
A Yawl is a two-masted sailing vessel with a tall main mast forward and a small mizzen (aft) mast stepped abaft the rudder post — that is, behind the point where the rudder is hung. The small mizzen is used more for balance and trim (helping steering and reducing helm load) than for driving the boat.
The yawl is distinguished from the similar ketch by the position of the mizzen: on a ketch the mizzen is forward of the rudder post and is a larger, more driving sail; on a yawl it is smaller and further aft. Yawls were popular as cruising and traditional sailing rigs. Understanding the yawl (and its cousins the ketch, sloop and schooner) is part of the traditional vocabulary of sailing-vessel types that underpins the language of the sea.
The yawl is part of the classic vocabulary of sail — and the fine distinction between a yawl and a ketch (where the little mizzen mast sits, relative to the rudder post) is exactly the kind of detail that defines traditional vessel types. It rounds out an understanding of two-masted rigs.
How is a yawl different from a ketch?
On a yawl the small mizzen mast is abaft (behind) the rudder post and used mainly for balance; on a ketch the mizzen is forward of the rudder post and larger.
What is a yawl's mizzen for?
Mainly balance and trim — helping steering and easing helm load — rather than driving the boat.