What is a modal tune?
Modes originated in the middle ages. They are an archaic way of representing scales. Each mode is a unique combination of whole steps and half steps. You can memorize the whole steps and half steps associated with each mode or you can relate modes to the familiar diatonic scale we use today. This discussion does the latter.
The diatonic scale of any Major key corresponds to the Ionian Mode. If we use the notes of the C Major diatonic scale, but start on the second note (or second degree) of the scale (D) rather than the first note (C), the series of whole steps and half steps that results is D Dorian. The Dorian mode is equivalent to starting on the second note (or degree) of the major scale. If we start on the third note (E) we get the E Phrygian. If we start on the fourth note (F), we get F Lydian. If we start on the fifth note (G) we get G Myxolydian. If we start on the sixth note (A) we get A Aeolian and so on.
Each major diatonic scale has 7 modes that correspond with the notes of that particular scale: Ionian (I), Dorian (II), Phrygian (III), Lydian (IV), Myxolydian (V), Aeolian (VI), Locrian (VII). The Ionian (I), Lydian (IV) and Myxolydian (V) are major sounding. The others are minor sounding. In practice the Locrian mode is rarely encountered.
Most Old Time modal tunes fall into the Dorian (II), Myxolydian (V) or Aeolian (VI) Modes. Modal tunes are generally played on the fiddle with standard GDAE tuning.