Tonality refers to the presence of a central key in a musical composition. If the music moves to a different key (see modulation), it is expected to return to the original key (called the tonic). Tonality gives the ear a "center," providing a context in which melody and harmony have meaning. Atonality, which emerged in some twentieth century, in music that lacks any central key (see atonal). Other modern composers have devised ways of working with tonal chords or characteristic tonal shapes without obeying the textbook rules of major- and minor-key tonality.