To my understanding, C major is the same as C Ionian, because both consist of the notes C, D, E, F, G, A, B, (C) in this order.
Contrast this with D Dorian, which consists of the same notes, but in a different order: D, E, F, G, A, B, C, (D).
Can there be big differences between C major and C Ionian?
The difference is simple. When you are talking about the major scale you are talking in a tonal context and when you are talking about Ionian you are talking in a modal context. You won't hear anybody use Ionian to describe a collection of pitches unless it is being used in a modal context. I'm not going to explain all the differences as this answer does, but I'll explain the biggest which is how the harmony of each is approached.
When you are talking in a tonal context, you're talking about functional harmony and this harmony revolves around the tonic-dominant relationship and takes advantage of the leading tone to establish a tonal center. When talking about something in C Major, you'll see most cadences will either be half (end on a dominant which is G in C), or perfect (go from V to I) and the progressions will be taking you either to or from the tonic. So the very familiar I - ii - V - I demonstrates going away from the tonic then returning via dominant.
When you are talking about a modal context you're still perceiving a home note (not tonic, that is a tonal idea), but the harmony is a lot more open and not defined by the tonic-dominant relationship although in Ionian and Lydian it will happen sometimes due to the leading tone being included in the mode. Modal progressions are greatly influenced by the melody and the progressions are much more just background than actually being used to drive the piece. An example of a progression that is more modal than tonal in nature in C Ionian is I-IV-I-iii-I or even I-vi-I-V-I as even in this context, even though we are using what could look like a dominant-tonic cadence, it is weakened by the progression constantly coming back to I either way.
A lot of pieces today aren't strictly tonal or modal and use concepts from both, but be aware of the difference as it's important to understand what's actually happening when someone says something is major verses Ionian.
The C major and C Ionian scales are the same scale, we have just taken to calling Ionian major. It's the same way that the c natural minor scale is the same as the C aeolian scale, we just call it by a different name. There is no difference in the scales.
The other answers here are basically correct: the scale notes of major and Ionian modes are the same, but major is more modern, and tends to use the I-V-I cadence pattern, while Ionian has other cadences possible- for instance, I-ii-I, as in Sumer is icumen in:
http://britishlibrary.typepad.co.uk/.a/6a00d8341c464853ef017615c0b763970c-pi
Trying to draw a hard and fast line between Ionian and major modes (yes, major is a mode too) is an exercise in futility, however.
I think in our current times we would say that they are the same. See other answers. Really the answer is "yes, but no".
We need to think historically. Where does the Ionaian mode come from? Early music? Gregorian chant? "Modal" music? The Ionian mode is an outgrowth of the Lydian mode for voice leading purposes. In F-Lydian there is a B-natural and in F-Ionian there is a Bb. It was common practice in early church and secular music to "raise" the fourth tone (B-natural) when ascending and "lower" when descending. But it is important to note that Lydian was actually more commonly used.
It's also incredibly important to note that the idea of Ionian and its relation to the Greek scales is completely made up by Mediaeval Europeans. We don't know what the Greek Ionian scale is, there are only loosely academic ideas, but nothing definitive.
So, back to Gregorian chant. The "modes" were actually just constructions and definitions of common scales used in songs that people were singing. Theory almost always follows practice. Rather than the 7 modes we think of now, there were 14. Ionian; hyper-Ionian. Dorian; Hyper-dorian. Etc, etc... These names were used because they defined the range and common cadences of each set of pitches. Ionian and hyper-ionian were indeed what we would call a major scale, but they were used in functionally very different ways.
First and foremost, to say define common chord movements in Ionian vs C-Major is denying the fact that Ionian stems from "pure" counterpoint and voice leading. Yes, in modern contexts we thing of modal harmony. However, If you dive into the works of say Palestrina, you will see how different he uses the Ionian and how different it is from a Major Scale. The major scale indicated tonality, and often the shifting of tonality, more in line with our modern ear.
You can look in Palestrina's music and see that while he only uses "the white keys" (with exceptions for voice leading), he cycles through many, many modes. He has work in E Phrygian, which shares all the notes of C Major, later he will move into A Aeolian, which shares all the same notes as C Major, and then into C Ionian. Of course A Aeolian is different than C-Major even though they share the same notes, but so is C Ionian. It's about the specific ranges, movement, and function of each scale. This is a very different musical world to our current one.
So history is important, but even more important is historiography. Modes are now used in many. many different contexts and this is because our relation to these historical constructs are constantly changing based on how they are interpreted. Right now, as you can see from the answers, there may not be a huge difference between C Major and C Ionian. However, historically the difference is substantial.
The C major scale is identical to C Ionian.
A piece in C Ionian will tend to restrict itself to the notes of the scale. A piece in C Major would be considered very simplistic if it did that!
A mode is monochrome. A key can be coloured in.
Modes are great. But they're self-contained, walled gardens. Keys are foundations, home bases, from which music wanders freely - but to which it generally returns.
There is no difference.
The term "Ionian" was invented in 1547 by a Swiss musicologist (Heinrich Glareanus) who decided for some reason that there should be 12 modes, rather than the traditional 8. The four that he added were two versions of the major scale and two versions of the melodic minor, which previously had never been called "modes" by the musicians who used them.
The reason for Glareanus invented two versions of each (i.e. "Ionian" and "Hypoionian") was for consistency with the traditional restriction that the compass of any particular mode should be strictly limited to one octave - though modern usage of "modes" no longer takes much notice of that idea.
Note: the ancient Greeks also used the term "Ionian" (or "Iastian") but that referred to a different theoretical concept from the notion of a "mode" that has been used in Western music for the past 1000 years or so, up to the present time.
It is not even known whether Heinrich had any accurate knowledge of ancient Greek musical theory. The earliest publication of an ancient Greek work on music was by Marcus Meibomius, about 100 years after Heinrich, and there was little research on the subject after Meibomius until the 20th century.
let's start from the beginning (not BC music history) but rather from the here and now and keep it simple:
(1) All music is "Modal" including all diatonic (Maj. and minor) based music, Maj. and min. act as the 2 "Parent" structures for all 7 C-Maj. Modes including the Ionian and the Aeolian Modes. There is a big difference between paying in the C Major Mode in comparison to playing in the C Ionian Mode- despite some close similarities, first off all- one has a key center and other does not.
(2) the C Ionian Mode ( like all the other 7 C Maj. Modes ) is technically limited to 1 octave, consist of only the 1st 5 authentic notes C-G ( C-D-E-F-G ) the modern version includes the 2 Hypo Ionian notes from G-C' ( A-B ) and begins and ends on the note C. A Mode, including the Ionian Mode like all the other 7 C-Maj. Modes has no key (tonal) center, all 7 C Maj.-Modes only have a home or reference tone / chord and only tend to be either Maj. or minor (Locrian being the exception and Mixolydian being a little bit of both). Up to this point or up to the 1st octave, C-Ionain and C Major are basically the same but also here one can see that there is already a major difference between C Ionian and C Maj.- albeit primarily in theory.
(3) Here comes the deciding differences (in practice) between C-Ionian and C-Maj. in this case (this point also applies to all the C Maj. Modes) it's not the notes in the mode that change, other than C Ionian it's simply the transposition of the C major scale up to the 1st octave, and the „Modal Chords“ that accompany a given mode and the relationship of the transposed scale tones to the Modal chords that change! In order to evoke a mode in a musical sense, one must create the given „Modal-Sequence-Motif“, this is different from a diatonic chord progression in that the "Modal Motif" is at best for some of the modes limited to just 2 main Chords that function as a support to bring a particular mode to the forefront, i.e. with C-Ionian this would / could be C Maj. to G Maj. I-V or C Maj. to F Maj. I-IV (of course there are a couple of other possibilities with this mode and or exceptions with some of the other modes) basically with only 2 chords (or any chordal arrangements with more than 2 chords that do not include any one of the strong and definite leading tones i.e. IV or V or VII / chords) one cannot establish any clear or definite tonal direction or key, we can only create a Mode with all of it’s implications.
Synopsis:
C Major Mode= (1) Has a key center with a true tonic chord (2) Use of the complete C major cadence (3) use of the entire C Major Scale (4) Not limited to any one octave or any certain beginning or ending tone or chord belonging to the C Major family (4) Does not have to resolve back to it’s tonic chord or tone (5) Can be tonicized (6) Can be harmonized (7) Belongs to the Major Keys
C Ionian Mode= (1) No definite key center with a quasi (I) chord as a starting point (2) Limited to the C Ionian modal sequence motif or modal chord structure (3) Technically limited to 1 octave use of only the modal scales tones from C-C’ (4) must return (or „resolve“ as it does not technically resolve in a harmonic tonal sense back to any particular chord in the modal sequence) to it’s 1st Modal Chord (5) Cannot be tonicized (6) Cannot be harmonized (7) Only tends towards the major tonality.
These are just a few of the substantial differences between playing in the C Ionian Mode as opposed to playing in the C Major Mode! The difference is considerable- otherwise there would be no reason for it's existence in the first place, of course both are going to sound Major or similar to a certain point, but only one of them is truly the Major Mode.
