Modal Scales
A brief explanation

Most of the music of western Europe and, by extension, European-colonized America, is based on scales that can be traced back to modes used in ancient Greece. While there is historical evidence of melodies using scales of five or six tones within an octave, most tunes still in use within that last two or three centuries use a scale of seven tones within an octave. Some tunes also include some "accidentals", that is, semi-tones not included in the usual scale sequence. This allows for the possibility of up to twelve tones within the octave although tunes having more than one or two accidentals are extremely rare.
The easiest way to visualize modal scales is to stick to the white keys on the piano keyboard. The most common mode in use is the Ionic or Major mode which is played by going from C - C on the white keys. The sequence of intervals is:
Whole tone - Whole tone - half tone - Whole tone - Whole tone - Whole tone - half tone
The other modes are derived by using the same sequence of intervals but starting at a different place or note on the keyboard.
The modes are:
Mode
Scale
 Intervals Frequency
Ionian (major)
C - C
 W-W-h-W-W-W-h common
Dorian
D - D
 W-h-W-W-W-h-W common
Phrygian
E - E
 h-W-W-W-h-W-W uncommon
Lydian
F - F
 W-W-W-h-W-W-h rare
Mixolydian
G - G
 W-W-h-W-W-h-W common
Aeolian (minor)
A - A
 W-h-W-W-h-W-W common
Locrian
B - B
 h-W-W-h-W-W-W rare


One confusing thing about modal scales is that while most (but certainly not all) melodies end on the tonic note of the scale, simply looking at the ending note does not always tell you in what mode the tune is set. A good example of this is that there are many tunes ending on D that are written by transcribers with two sharps in the key signature even though the seventh notes (C) are not sharp and must be written as accidentals. This means that the tune is in the Mixolydian mode and the key signature should contain only one sharp. To determine the correct mode you have to look at the interval sequence instead of just the ending note.
Another thing to remember is that the traditional players were ear players. Most of them could not read musical notation and were not concerned with key signatures or modes. If they were playing harmony they let the tune end on whatever note it did and they did not have our modern "educated" compulsion to resolve the ending to a (usually) major chord.