A ketch is a sailing craft with two masts: a main mast, and a shorter mizzen mast abaft (rearward) of the main mast, but forward of the rudder. Both masts are rigged mainly fore-and-aft. From one to three jibs may be carried forward of the main mast when going to windward. If a ketch is not rigged for jibs it is called a cat ketch, sometimes called a periauger. On older, larger ketches the main mast may in addition carry one or more square rigged topsails. A ketch may also carry extra sails, see below.

The lowest fore-and-aft sail on the main mast is called the mainsail, while that on the mizzen is called the mizzen sail. These may be any type of fore-and-aft sail, in any combination. The Scots Zulu, for example, had a dipping lug main with a standing lug mizzen.

The ketch is popular among long distance cruisers as the additional sail allows for a better balance, and a smaller more easily handled mainsail for the same overall sail area. It also allows sailing on mizzen and jib only without introducing excessive lee helm, and in an emergency can be quite well steered without use of the rudder. The ketch is a popular rig in northern European waters where sudden increases in wind strength sometimes requires a rapid reefing: the mainsail can be dropped, reducing sail and leaving a balanced sail-plan with jib and mizzen set.

Running before the wind or reaching across the wind, a ketch may carry extra sails such as a spinnaker on the main mast, and a spinnaker or (mizzen staysail) on the mizzen mast.