In the middle position the top wheel pinches the string, resulting in a natural, giving the scale of C major if all pedals are set in the middle position.
From the en.wikipedia.org
In the bottom position another wheel is turned, shortening the string again to create a sharp, giving the scale of C-sharp major if all pedals are set in the bottom position.
From the en.wikipedia.org
Many of the musical activities actually tie the C major scale to the keys found in each row of the keyboard.
From the usatoday.com
The dominant key of C major is G major so this secondary dominant will be the chord of the fifth degree of the G major scale, which is D major.