Complement and Compliment

When you “complement” (with an “e”), you add to or complete something:

The raspberry jam really complements the peanut butter in this sandwich.

(The raspberry jam in combination with the peanut butter makes this sandwich better.)

When you “compliment” (with an “i”), you flatter someone or something:

The raspberry jam often compliments the peanut butter.  The other day I heard it say, “Peanut butter, you sure are the smoothest and creamiest spread I have ever seen.”

(The raspberry jam speaks with the peanut butter and makes it feel good about itself.)

Resources

Paul Brians’s Common Errors in English Usage has an extensive list of errors that is worth perusing.  At the bottom of the Oxford Dictionary’s definition for “complement,” there is also a good explanation of the difference between “complement” and “compliment.”

Proprofs has a good quiz.