What is the primary function of a slur in music notation?

The primary function of a slur in music notation is to connect notes smoothly. A slur is depicted as a curved line that spans over a series of notes, signaling to the performer that these notes should be played in a legato manner, meaning they should be connected without any perceptible interruption or break in sound.

When a musician sees a slur, it prompts them to transition seamlessly from one note to another, enhancing the expressiveness and lyrical quality of the music. This technique is often used in various styles and genres to create a more fluid and cohesive sound, essential for melodic lines that require greater musicality.

The other choices do not describe the primary function of a slur. For instance, indicating loud playing relates to dynamics, and marking the end of the music refers to a different type of notation entirely. Similarly, indicating a rest period also concerns a different notation concept, emphasizing silence rather than continuity. Thus, connecting notes smoothly is indeed the correct identification of a slur's role in music notation.