A question about modes

[Kolutic]

Hey guys, I'm new to the modes and am still unsure why some song would be labeled to be, for example, composed in G Mixolydian when it uses all the same notes as a C Major/Ionian scale. Why use two different names for two scales that have the same notes?

Sure, if you play them up and down, there is a difference in sound, but in a song, you don't just walk the scale up and down, you take the notes and juggle them around to create a more complex melody, so from the example above, if I say the song is G Mixolydian, is it or is it not (and why so) the same as if I said it is in C Major. How do I know, and how does it exactly matter, if the song was composed in C Ionian or D Dorian, E Phrygian, etc.?
I know this must sound nooby it's just that I don't see clearly what difference there is.

[Alan Green]

Noteboat knows all this in detail, but:

If you play C Major (Ionian) you get a scale with a leading note of B which is one semitone below C

If you play G Mixolydian, you get a scale in C Major which is an identical scale to G major but with an F-natural (instead of the leading note of F#) which is a whole tone below G

So, the difference is in the order of the intervals.

[guitarforlife]

Because the tonal center is G and not C.

[NoteBoat]

Yep, guitarforlife has it - but 'tonal center' can be a tough thing to get your head around.

The tonal center of a song is "home base". It's the point where your melody reaches full stop. But there's really no objective measure to tell you what the tonal center is... you just have to hear it and know that's the one.

Some people will tell you that it's the last note of the melody. That's often true, but not always. A small percentage of tunes will leave you hanging on some note that doesn't seem fully resolved.

Some people say it's the first note. That's usually wrong, because starting on the tonal center means you'll need more 'oomph' to get an interesting melody. It's true in some cases, like "Over the Rainbow", but when a tune starts on the tonal center, it's almost always followed by a big leap (as it is in that tune). Much more common is starting on some other note - for example, "Happy Birthday" starts on the fifth and ends on the tonic, or tonal center.

Some people say it's the note that's heard most often. Sometimes that's true, sometimes it isn't. The fact is, for every 'rule' I've ever heard, I can compose an example that breaks the rule - I think I did that in a thread on modes here 7 or 8 years ago.

As Alan said, the real thing about modes is the relationship of intervals to the tonal center. Our ears are used to hearing songs in major (Ionian mode) or minor (sometimes the Aeolian mode, but there are lots of minor scales). If something sounds a bit off, it may be because the tune is modal.... you're expecting major or minor scale intervals from the tonal center, and one or two notes seem a bit off.