Twain is an old-fashioned word meaning two. You'll most often see it in the famous phrase “never the twain shall meet,” which means two things are so different they can never come together or agree. When two friends have completely opposite opinions about which sport is best, you might say never the twain shall meet about their basketball and soccer preferences.

Today it survives mainly in that one phrase and in the pen name of Samuel Clemens, who called himself Mark Twain. He took the name from riverboat terminology: when people measured the depth of the Mississippi River and called out “mark twain,” they meant the water was two fathoms (twelve feet) deep, safe enough for the boat to pass.

You might encounter twain in older books or poetry, where it simply means a pair or two of something. A knight might cleave a sword in twain, splitting it into two pieces. While people rarely talk this way anymore, understanding twain helps you read classic literature and recognize when modern writers use “never the twain shall meet” to describe two things that seem impossible to reconcile.