ketch usually refers to a sailboat with a particular mast configuration:
  1. A mainmast somewhat forward of amidships, and
  2. A smaller mizzenmast just forward of the helm.
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Ketches are frequently confused with yawls; however, a yawl is a slightly smaller boat with its (smaller) mizzen aft of the helm.   Different designs of ketches have vertical stems or raked masts.

"Ketch" is a Dutch word; these boats are known in Scandinavia by "galeas".   They were used for centuries along the Northern Eurpoean coast as trading vessels.