Why Do Ties and Slurs Look Alike but Mean Different Things?

A tie and a slur are both drawn as a curved line, but they do different jobs. A tie joins two adjacent notes of the same pitch. The second note is not sounded again; the written durations are added into one sustained event. A crotchet tied across a barline to a quaver is therefore heard as one note lasting a beat and a half, not as two attacks.

A slur usually spans different pitches and asks for them to be played within one connected gesture or phrase. On the piano, it commonly calls for minimal gaps between notes and a slight release at the end of the slur. On a string instrument, it may also determine which notes belong in one bow stroke. It does not cancel the later notes: each retains its own beginning.

The shared shape is not simply ambiguous notation. Context does much of the work. A short curve between identical pitches is normally a tie; a curve across different pitches is normally a slur. When repeated notes sit beneath a longer curve, performers must also consider the voice, instrumental practice and edition, because the mark may indicate phrasing rather than tied duration.

The decisive audible distinction is this: a tie changes the internal structure of rhythm without adding a new attack, while a slur preserves each attack but changes how the notes connect. A pianist who mistakes a tie for a slur plays an extra note; one who mistakes a slur for a tie may omit a note. Reading notation is not just recognising shapes, but understanding what a sign does in musical time.

https://openmusictheory.github.io/rhythmicValues.html
https://human.libretexts.org/Bookshelves/Music/Music_Theory/Understanding_Basic_Music_Theory_(Schmidt-Jones)/03%3A_Notation_-_Style/3.02%3A_Articulation