You already know two modes without realising it. The major scale is Ionian mode. The natural minor scale is Aeolian mode. Modes are what happen when you play the same seven notes but treat a different note as home. The pitch collection is unchanged, but the interval pattern and harmonic centre are heard relative to the new tonic.
Modes: same notes, different centre
Take the C major scale (C D E F G A B). Now play those exact same notes, but start and resolve on D instead. The pattern of tones and semitones changes relative to your new root, giving a different scale pattern. That's D Dorian: the same pitch collection as C major, heard from D with a minor third and natural sixth.
The seven modes of C major
Mode I: Ionian (the major scale)
This is the major-scale mode, with a major third, perfect fifth, and major seventh above the tonic. Pattern: T-T-S-T-T-T-S.
C Ionian (C major) - the familiar starting point
Mode II: Dorian
A minor mode with a natural sixth instead of the lowered sixth found in natural minor. That degree distinguishes Dorian from Aeolian when the tonic and surrounding harmony make the comparison audible. Pattern: T-S-T-T-T-S-T. Semitones: 0, 2, 3, 5, 7, 9, 10.
D Dorian - same white keys as C major, centred on D
Mode III: Phrygian
A minor mode with a lowered second (b2), one semitone above the root. That close interval is a defining colour when it is exposed against the tonic. Pattern: S-T-T-T-S-T-T. Semitones: 0, 1, 3, 5, 7, 8, 10.
E Phrygian - that b2 (F, one semitone up) is the signature sound
Mode IV: Lydian
A major mode with a raised 4th (#4). That interval distinguishes it from Ionian and can be highlighted by a melody, sustained tonic, or chord that contains the raised fourth. Its effect depends on the voicing and surrounding harmony. Pattern: T-T-T-S-T-T-S. Semitones: 0, 2, 4, 6, 7, 9, 11.
F Lydian - the raised 4th (B natural) gives it that floating feel
Mode V: Mixolydian
Major with a flat 7th. It's the sound of dominant 7th chords and has a bluesy, laid-back feel. Pattern: T-T-S-T-T-S-T. Semitones: 0, 2, 4, 5, 7, 9, 10.
G Mixolydian - same as G major but with F natural instead of F#
Mode VI: Aeolian (natural minor)
Another one you already know. The natural minor scale is Aeolian mode. Pattern: T-S-T-T-S-T-T. Semitones: 0, 2, 3, 5, 7, 8, 10.
A Aeolian (A natural minor) - the standard minor sound
Mode VII: Locrian
The most unstable mode. It has both a b2 and a b5 (diminished fifth from root), making it difficult to feel settled. Rarely used as a key centre, but important in theory. Pattern: S-T-T-S-T-T-T. Semitones: 0, 1, 3, 5, 6, 8, 10.
B Locrian - the diminished 5th makes it inherently unstable
Comparing modal pitch content
A common comparison orders modes by how their scale degrees sit against the major scale. Raising or lowering particular degrees changes the interval content and can affect perceived brightness, tension, and stability; the result also depends on melody, register, voicing, and arrangement.
Try it
Choose one mode from the C-major notes and write its notes in root order. Before playing, predict its signature degree: Dorian's natural 6, Phrygian's b2, Lydian's #4, Mixolydian's b7, or Locrian's b5.
Play the scale beginning and ending on that mode's root, then play the same notes beginning and ending on C. Note how the tonal centre changes the result.
Key takeaway
Seven modes from one major scale, each starting on a different degree. Three are major-type (Ionian, Lydian, Mixolydian), three are minor-type (Dorian, Phrygian, Aeolian), and one is diminished (Locrian). You already knew two of them as the major and natural minor scales.
Next: how modes are actually used in different genres of music.
Generate in any key or scale
Starts produces MIDI in your chosen key and scale - major, minor, modes, and more.