Tie
Connects two notes of equal pitch into one sound; the durations are added. Not to be confused with the slur.
What it means
The tie (German Haltebogen) connects two immediately successive notes of the same pitch into a single sound. Their durations are added: the note is attacked once and held over both values.
Strictly to be distinguished from it is the slur (German Bindebogen), which looks the same externally. It connects notes of different pitch and means not prolongation but legato — the notes are strung together without a gap but attacked individually. The rule is simple: the same pitch means a tie, different pitch means a slur. With chords, a curve between two notes of the same sonority can act as a tie while other voices are struck anew.
The tie is the means of notating durations that cannot be represented with a single note value, and of holding notes across the bar line.
In the notation
The curve runs from notehead to notehead, on the side facing away from the stems. It can lead across a bar line and across a change of line; with more than two notes, a separate curve is set for each pair. An accidental applies only to the first note and continues to act across the tie, even if the next bar has begun.
Musical use
Tie and dotting achieve something similar, but not the same: the dot lengthens by half of its own value, the tie allows arbitrary sums. Across the bar line the tie is the only way to let a note go on sounding. Syncopations often arise by a note being attacked on a weak beat and held across the following strong one.
Other names
Haltebogen is the German designation; in English it is called tie, and a tied note tied note. Since the legato slur is also called a slur in English, English distinguishes the two more precisely in language than German does.
Performance practice
Do not attack the second note anew — this is the most common mistake and makes two sounds out of one. Count out the added duration completely, especially across the bar line. With every curve, first check the pitches to distinguish tie and slur. And watch for accidentals that apply across the curve: after the bar line they are easily overlooked.