A curved line joining two or more notes of different pitches indicates a slur. Only the first note has a definite beginning. This is common in singing where two or more notes may occur while singing a single syllable.
In the first bar of the example that follows, the syllable ah occurs on two consecutive notes and is articulated on each note. In the next bar the same two notes are connected by a slur, so ah is sung again but then "slides" or "glides" down to the second note. In the third bar, the syllable ah occurs for the last time, this time covering three notes connected by a slur. Once again, ah is only articulated on the first note, and then glides or slides to the other notes.
A good example of the way this occurs in a song is the opening phrase of Silent Night, which you learned in the previous lesson. It is shown next with slur markings.
To keep the notation uncluttered, slur markings are often left out of sheet music. You can usually tell where they occur because a syllable will stretch over two or more notes, and this will be indicated in the lyrics by a hyphen (e.g., Si – lent night).