Strictly speaking we don't. There are many other "modes". Each mode has a different template for the pattern of Tones and Semitones between each note of the scale. The pattern for the Ionian Mode is: TTSTTTS Which is identical to the pattern for our Major scale today. The pattern for the Dorian Mode is: TSTTTST This can be regarded as a minor mode (due to the minor 3rd).by Jenks - Music Theory Questions and Answers