A quarter note is a basic musical note that represents one beat in a piece of music. It is typically notated as a filled-in oval with a straight stem. In common time signatures, such as 4/4, four quarter notes make up one measure. This makes the quarter note an essential building block for rhythm in various musical styles. In sheet music, quarter notes can be combined with other note values, like eighth notes and half notes, to create different rhythmic patterns. Musicians often use quarter notes to maintain a steady tempo, making them crucial for both beginners and experienced players in understanding rhythm and timing.