I've learned all about modes, but there's something that confuses me.
The G major scale contains the exact same notes as the A Dorian scale. So what determines which of those two scales you are playing?
I know if you played a low G or a low A, then we'd hear that as the root and the matter would be settled. But without holding on the root, it seem like the mode is very much up in the air
Every mode has a tonal center, a point of greatest stability within the overall sound of the mode. So a piece in G major will tend to move toward G as a pitch goal; whereas, a piece in A dorian will gravitate toward A.
If one in looking at a written score, a good starting point in determining the mode is to look at the key signature and then at the final sound of the piece. Since a tonal center is the "at rest" note for the mode, it's reasonable to expect the piece to end on that pitch. So a piece in G major will have one sharp and end on G (or a G major chord), and a piece in A dorian will also have one sharp but end on A (or an A minor chord).
A mode is a collection of notes which have a certain set of relationships to each other. Scales are the abstract representation of modes: the set of notes, ordered. The steps of each modal scale has a unique set of intervals. Major, for example, is represented by a scale whose intervals are WWHWWWH. Dorian, by contrast, is represented by a scale whose intervals are WHWWWHW.
When a piece is written "in a mode", it will emphasize the unique relationships within that mode. For example, major emphasizes the half step between the seventh and eighth notes. Dorian, to distinguish itself from major, can emphasize the whole step between those same two scale degrees. The major third between the first and third notes of the major scale can be contrasted with the corresponding minor third in the dorian scale.
By ear
Distinguishing a mode by ear relies on being able to identify the tonal center (not the note name, but the pitch that sounds like the "target" pitch) and the characteristic sound of that mode.
By score analysis
Distinguishing a mode analytically also relies on being able to identify the tonal center and noticing the predominant pitches used in the piece. A piece that has C as a tonal center, and mostly uses "white keys", but emphasizes F#, could be reasonably determined to be in C lydian.
The G major scale contains the exact same notes as the A Dorian scale. So what determines which of those two scales you are playing?
mode = set of notes + tonic
The sense of mode is somewhat subjective and depends on where the music makes the listener feel the tonal center to be. Rhythms and sense of pulse and meter, affect that as well as pitches! Meter means, where the "one" beat is and which notes are strong vs. weak - which can be subjective as well. It is entirely possible for a player to "use" the scale of a mode, but FAIL to produce the feeling of that mode. It's not what notes you play, it's how and when you play them. Playing notes from a scale randomly will not create the right modal feeling (except by accident). Using only the white keys of the piano, you can create many different modal feelings. The fact that the keys are white doesn't guarantee anything.
without holding on the root, it seem like the mode is very much up in the air
Exactly. You have to practice it. Here is a longer answer.
Why do modes sound so different, although they are basically the same as a mode of another scale?
(This whole question is yet another dupe, it's being continuously re-asked over and over again by different people using slightly different wordings, so why not have it all out there, just in case someone found one of the answers with THAT particular wording that the person happens to be using.)
Tonal music is called tonal, as it relies on one particular note as its 'home' note - its tonic - hence tonal.
The majority of tonal music is somewhat like a journey. Our journeys tend to take us from 'home', visiting wherever, and eventually, arriving back at square one - home again. We may well make several journeys during that period, but each will end finally back at home.
Using that analogy, any piece of tonal music will, of necessity, have a 'home'. That's when modal music becomes recognisable. Of the 7 modes produced by use of the notes available from one 'parent' key (thus, its scale notes), each will have its own tonic, or home. Each will use that note to return to, as a temporary rest place, throughout, and finally feel finished on that same note.
Thus, the other notes involved will have particular relationships with that tonic. In Ionian mode, there is always a leading note, one semitone below the tonic, which when heard often begs that the tonic will follow, and if it doesn't (as sometimes happens), resolution is denied, producing a feeling that the journey's not over yet.
Using those same 7 notes, but in Dorian mode, there's no leading note as such, so that feeling cannot be invoked. There's also the factor that the 3rd of any mode will be either m3 or M3, giving a minor or major feel to the piece, whenever that particular note is heard. One of the reasons Aeolian is considered, along with Dorian in jazz, as the prominant minor mode.
To establish a mode in a piece, then, the tonic needs to be a prominant note, the leading note will be used to herald its arrival (or not, if there isn't one!), the 3rd of the mode will underline whether it's a minor or major mode, and the triads involved (all the same in all modes from the same parent) will be used in such a way that the tonic triad becomes sensed as the home triad.All that, and more, is why the Locrian mode is probably least used, as it finds it hard to fulfill any, or many, of those criteria...
