Tonic is important because it says where the tune is going. Mode is important because it says how the tune is going to get there.
If a tune is in an open key (no white notes on the harpsichord) then in the major (or "Ionian") mode the obvious harmony is the "three chord trick" C, G7 and F. The final sequence will be G7 => C If it's in D dorian then you will hear Dm, C and very occasionally either G or Bm. the final sequence will be C => Dm If it's in E phrygian and is a flamenco piece (perhaps "phrygian major") you'll hear a characteristic ending sequence of "Am" => G7 => Fmaj7 => E If it's in the minor phrygian then I haven't a clue and the same applies to F lydian. If it's in Gmix then you'll hear mainly G and F (Old Joe Clark) and the odd C, finishing with F => G. In A aeolian you'll hear Am, C, and maybe a G or F, probably ends G=>Am but I'm not so sure about this one. If it's in A minor then you'll hear Am, E or E7, and the odd G, C or F, finishing E7=>Am For B locrian I have no idea. As Robert Bley-Vroman wrote, it tells you what to shout to the guitarist. e.g. "It's one of them Em-D tunes.". Contrast this with my "Concise dictionary of music" which says "By the end of 17th cent. exclusive use of 2 modern modes, major and minor was established". (Clearly Old Joe Clark and Drowsy Maggie are simply wrong and need to be re-written properly). To summarise it so you can see it at a glance, here it is as a table. The final sequence is the first two chords in reverse order e.g. G7=>C. K:Cmaj C G7 F K:Ddor Dm E (G, Bm) K:Ephr E Fmaj7 G7 Am K:Gmix G F (C) K:Aaeo Am C (F G) K:Am Am E7 (G C F) and in case you think that they look different just because they all have different tonics, here it is transposed to the same tonic: K:Cmaj C G7 F K:Cdor Cm Bb (F, Am) K:Cphr C Dbmaj7 Eb7 Fm K:Cmix C Bb (F) K:Caeo Cm Eb (Ab Bb) K:Cm Cm G7 (Bb Eb Ab) So, to repeat, the tonic tells you where the tune is going and the mode tells you how it's going to get there - the notes that are likely to turn up in the melody, the likely harmony and the likely harmony sequence. And you don't need any maths, you just need an ear for the music. It's pretty clear that some classically trained musicians have had that trained out of them. (Is it like Japanese not hearing the difference between an English L and an R?). John Chambers is of course correct, there are a lot more modes than this and some of them (Lydian, Phrygian minor and Locrian) are so rare that they might just as well have been treated as odd-balls and written out with accidentals. But to refer to a D-dor tune as D minor tells the guitarist to listen for C#-natural leading notes and the chance to play A7 chords. Bryan Creer had written: "...the "ignorant peasants" were perfectly capable of singing their songs without knowing anything about modes..." That of course is stretching the phrase "knowing anything about" to breaking point. They knew the scales and they knew how to harmonise them. They just didn't know the Greek names. Shakespeare's Theorem applies: "A mode by any other name will sound as sweet." Laurie To subscribe/unsubscribe, point your browser to: http://www.tullochgorm.com/lists.html
