Legato slur
Slur that joins notes into legato.
What it means
The legato slur requires that the notes it spans be played bound: one note passes directly into the next, without interruption and without a new attack. It thus concerns articulation, not duration. In this it differs from the tie, which draws two identical pitches together into a single long note — both look the same; the difference lies solely in whether the connected noteheads designate the same pitch or different ones.
In the notation
The slur is drawn above or below the noteheads, opposite to the stem direction, and begins and ends exactly at the notes it encloses. With a slur over a few notes the intent is unambiguous; if it extends over whole bars, it must be checked in each case whether binding or phrasing is meant. The distinction between legato slur and phrase slur is not marked in the notation and results from length and context. In older sources the placement of slurs is often inconsistent, and editors add missing slurs — such additions are marked in urtext editions.
Musical use
For string instruments the slur has an additional, very concrete meaning: it indicates what is to be played in one bow stroke, and thereby directly determines the bowing. For wind players it governs the tonguing — bound means not attacking anew. On the piano, where every note is struck individually in any case, legato means that the key is released only when the next one sounds; the fingering must be arranged accordingly. In all cases, binding usually involves a slight emphasis on the first note under the slur and an easing off on the last.
Performance practice
With every slur, check first whether the connected notes have the same pitch — then it is a tie and is not attacked anew. Arrange your fingering so that the binding is physically possible, and practise the passage first without the slur marking, to secure the notes, then deliberately bound. Watch the end of the slur: the last note is eased off slightly, and after it something new begins.