section_path: Elements/Harmony
section_name: Harmony
paragraph_text: Harmony
  
  Harmony refers to the "vertical" sounds of pitches in music, which means pitches that are played or sung together at the same time to create a chord. Usually, this means the notes are played at the same time, although harmony may also be implied by a melody that outlines a harmonic structure (i.e., by using melody notes that are played one after the other, outlining the notes of a chord). In music written using the system of major-minor tonality ("keys"), which includes most classical music written from 1600 to 1900 and most Western pop, rock, and traditional music, the key of a piece determines the "home note" or tonic to which the piece generally resolves, and the character (e.g. major or minor) of the scale in use. Simple classical pieces and many pop and traditional music songs are written so that all the music is in a single key. More complex Classical, pop, and traditional music songs and pieces may have two keys (and in some cases three or more keys). Classical music from the Romantic era (written from about 1820–1900) often contains multiple keys, as does jazz, especially Bebop jazz from the 1940s, in which the key or "home note" of a song may change every four bars or even every two bars.