Diatonic means within the scale or within the key. Each major scale has seven tones that begin with the formula WS – WS – HS – WS – WS – WS – HS form the friendly character of major. Layering thirds (the basis of harmony) to form chords results in specific chords for each of the seven degrees, depending on the key. The most common is the major scale as an example, as it forms the basis of many musical works. To illustrate this, let's take the E major scale:

Triads on the major scale steps

Wenn ich durch die Terzschichtung nur leitereigene Töne verwende, bilden sich über die sieben Stufen in jeder Dur-Tonart immer folgenden Drei–, Vier– und Fünfklänge:

Scale Degree	Triad - Four-tone - Five-tone	Classical Label
First Degree (I)	Maj – Maj7 – Maj9	Tonic
Second Degree (II)	Min – Min7 – Min9	Supertonic
Third Degree (III)	Min – Min7 – Min9	Mediant
Forth Degree (IV)	Maj – Maj7 – Maj9	Subdominant
Fifth Degree (V)	Maj – Dom7 – Dom9	Dominant
Sixth Degree (VI)	Min – Min7 – Min9	Tonika-Parallele
Seventh Degree (VII)	Diminished – Min7b5 – Min9/b5	Verkürzter Dominantseptakkord

The function of the step chords (according to frequency of use in popular music)

Jeder dieser Dreiklänge hat in der Harmonielehre eine bestimmte Funktion, die das harmonische Fortschreiten bestimmt:

• Tonic (I)Home base. Most stable,
• Subdominant (IV): a chord that feels familiar.
• Dominant (V): a chord that has the highest tension and usually resolves to the tonic or to .
• Parallels (II, III, VI): these chords support the main functions and offer harmonic alternatives.
• Leading Tone (VII): a very tense chord that is rarely used