Music theory starts with major scales.

Musical alphabet = ABCDEFG ... ABCDEFG ... Etc.

Scale = Sequence of the musical alphabet.

Major Scale = Eight-letter sequence of the musical alphabet with the following intervals between scale tones:
 

Interval = Musical distance between two tones, two piano keys, two musical pitches.

H = Half-step interval = Two adjacent tones.

Ex: C - C#/Db = Half-Step Interval

Tones:	C	C3/Db
	White Key	Black Key

Ex: B/Cb - C

Tones:	B/Cb	C
	White Key	White Key

W = Whole-step interval = Two half-steps = Three Adjacent Tones

Ex: F - G

Tones:	F	F#/Gb	G
	White Key	Black Key	White Key

 
Sharp Sign: # = Next Adjacent tone to the right or higher in pitch than the original tone.
Ex: C - C# =    C    C#

Flat Sign: b = Next adjacent tone to the left or lower in pitch than the original tone.
Ex: Cb - C =    B/Cb C

Scale degrees are the numbers of the tones of any major scale.
 

C Major Scale:	C	D	E	F	G	A	B	C
Scale Degrees:	1	2	3	4	5	6	7	8
	Root (R)							Octave

Minor scales are related to major scales as variations of the major scales.
 

C Natural Minor Scale:	C	D	Eb	F	G	Ab	Bb	C
Scale Degrees:	1	2	b3	4	5	b6	b7	8
	Root (R)							Octave

 

C Harmonic Minor Scale:	C	D	Eb	F	G	Ab	B	C
Scale Degrees:	1	2	b3	4	5	b6	7	8
	Root (R)							Octave

Thus, when a student is learning his minor scales, it is easy for him to learn minor scales as variations of the major scales than as separate additional scales.

A key is the scale upon which a song is centered.
Ex: Key of C Major = Song is centered upon the C major scale.
Ex: Key of F Minor = Song is centered upon the F Natural or Harmonic Scale.

A song is centered upon a scale when most of its melodic tones and chords are found upon the scale, and, in particular, if there are no moduclations into other keys, the final chord is the chord built upon the root of the scale of the key.

A key-scale is the scale built upon the root of the key.
Ex: Key of C Major: Key-Scale = C major scale.
Ex: Key of F Minor: Key-Scale = F minor scale.

A chord-scale is the major scale of the root of any chord.
Ex: C major triad: chord-scale = C Major.
Ex: F Minor Triad: Chord-scale = F major.
Ex: G Seventh Tetrad: Chord-scale = G major.

NOTE: Since there are no clear and obvious designations of four-note chords, five-note chords, six-note chords, and seven-note chords, the following schema is offered:

3-Note Chord = Triad
NOTE: The traditional definition of “triad” as any combination of three tones played together or as an arpeggio used to support, or harmonize with, a melody is now to be replaced with the definition of triad to be a three-note chord.
4-Note Chord = Tetrad
5-Note Chord = Pentad
6-Note Chord = Sextad
7-Note Chord = Septad
 

Chords (chord types) are defined by chord formulas derived from the chord-scale (remember that the chord-scale is always the major scale built upon the root of the chord and not the key-scale):
Ex: Chord Formula: Major Triad = R-3-5
Ex: Chord Formula: Minor Triad = R-b3-5
Ex: Chord Formula: Dominant Seventh Tetrad = R-3-5-b7
Ex: Chord Formula: Minor Seventh Tetrad = R-b3-5-b7

Chord Numbers: Chords can be numbered according to the position of their roots upon the scale degrees of a scale.

Ex: Key of C Major:

C Major Scale:	C	D	E	F	G	Ab	Bb	C
Scale Degrees:	1	2	b3	4	5	b6	b7	8
Chords:	C	Dm	Dm	F	G(7)	Am	Bdim	C
Chord Numbers:	I	IIm	IIIm	IV	V(7)	VIm	VIIdim	I

From this philosophy of music theory all other music concepts, principles and techniques are derived.

To me, there is something goofy about teaching chords as stacked thirds.

For example, a sixth chord (R-3-5-6) can be labeled by traditionalists as a seventh chord with a diminished seventh (R-3-5-bb7), but this confuses the function of the sixth chord to be a stable chord which does not demand a resolution whereas a true diminished seventh chord (R-b3-b5-bb7) is a restless chord demanding a resolution.

While it is true that all chord-tones can be thought of as stacked thirds, nevertheless music theory works better/best when approached in terms of scale degrees.

Ex: C Major Thirteenth Chord: R  3  5  7  9  11  13

Ex: When we talk of chord-tones we talk in terms of roots, third, fifths, etc., and not “first stacked third,” “second stacked third,” etc.

Ex: When we talk of sixth chords we are talking about triads with the added sixth scale degree and not the added thirteenth scale degree or the “sixth stacked third.”
Ex: C Sixth Chord =  R-3-5-6.

Thus, from my 35 years experience teaching music and music theory, teaching music theory by means of scale degrees is the best method.