The English word "chord" derives from Middle English "cord", a back-formation of "accord" in the original sense of "agreement" and later, "harmonious sound". A sequence of chords is known as a chord progression or harmonic progression. These are frequently used in Western music. A chord progression "aims for a definite goal" of establishing (or contradicting) a tonality founded on a key, root or tonic chord. The study of harmony involves chords and chord progressions and the principles of connection that govern them.