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A slur is a symbol in Western musical notation indicating that the notes it embraces are to be played without separation. This implies legato articulation, and in music for bowed string instruments, it also indicates the notes should be played in one bow; and in music for wind instruments, that the notes should be played without using the tongue to rearticulate each note (see tonguing). In guitar music, the slur indicates that the notes should be played without plucking the individual strings, i.e. hammer-ons and pull-offs. In vocal music, slurs are usually used to mark notes which are sung to a single syllable (see: melisma).
When two instruments written on the same staff both have slurred phrases with the same note values (e.g., clarinets playing in thirds) it is customary to have two sets of slurs, though in some scores just one set is used and it is understood to apply to both of the instruments.
The slur is not to be confused with two other similar musical symbols. The tie is a curved line that links two notes of the same pitch to show that their durations are to be added together. The phrase mark is a curved line that extends over a passage which is visually indistinguishable from the slur, and indicates that the passage is to be interpreted as a single phrase.
A slur is a curved line used to indicate that notes should be played smoothly (legato) and in one breath. The notes are of different pitches and can be of different types. A slur can extended over two or several notes at a time, sometimes encompassing several bars of music. In that latter case composers are known to demand slurs which are near to impossible to achieve for the normally endowed singer (or player); presumably in that case the composer wishes to emphasise that the notes should be sung (or played) with as much legato as possible or rather the "impression" should be given.