Harmony can be thought of as the notes that support and add character to a melody. The basic building blocks of harmony are chords. A chord is a group of three or more notes played simultaneously (e.g., strumming on a guitar).
There are many different types of chords, the most common being the major chord. All major chords contain three notes, taken from the major scale of the same letter name. These three notes are the 1 (first), 3 (third) and 5 (fifth) notes (degrees) of the major scale, so the chord formula for the major chord is: 1 3 5. If these notes are taken from the C major scale the chord is a C major chord, usually just called a C chord. Chords are represented by symbols placed above the vocal melody on sheet music. The symbol for a C chord is the letter C. The symbol for a D (major) chord is the letter D. The symbol for an E flat chord would be Eb.