Sentence:  A few types have a portion of the face letter projecting over the body, as in the letter f ; this projection is called the <WSD>kern</WSD>, and in combination with other letters the projecting part generally extends over the next letter, as in fe. In those combinations, wherein the <WSD>kern</WSD> would come in contact with another letter, compound types are cast, as in the case of ff, fi, fl, ffi, ffl. 
Meaning of the word marked with <WSD> tags:  (letter)  Any part of a letter which extends into the space used by another letter.