The note Middle C and the note B, as shown on the Treble Staff and the Bass Staff.
In Book 2, Lesson 9, this B note was written on the treble staff and you played it with your right hand. In this lesson the B note is written on the bass staff and you play it with your left hand thumb.
Play the C major scale with your left hand. To play the scale smoothly, you will need to use a crossover. On the way up the scale, move your second and third fingers over your thumb. On the way down the scale, move your thumb under your second and third fingers.
In this piece, the right hand plays the C major scale and then the left hand plays the scale an octave lower. Scales are used to make lots of melodies, so it important to be able to play them well. When you play a scale or melody, play them legato and imagine your hands are singing the notes.