Intervals

[linglingwannabe]

 Why are both C to Db and C to D considered seconds if the first two are 1 half-step apart and the other two are 1 whole-step apart?

[j_tour]

Why are both C to Db and C to D considered seconds if the first two are 1 half-step apart and the other two are 1 whole-step apart?

Well, the quick answer is that they're not considered the same way.  One's a minor second, and the other is a plain second interval. 

It's like why is a minor triad not considered a major triad with an altered note.  It's just not.

All right, here's a brief edit:  I have to be out the door in a few minutes, but consider this, right?  So, all the common practice period hamonies and establishing of tonality, these intervals and chords didn't just arrive out of thin air. 

Maybe this you can do:  where did the minor second interval come from, exactly?  Hint, look at the modes of the major key, or the modes of the harmonic minor.

I really do have to get out the door, but that seems about right to me:  where'd these intervals come from, anyway?  I'd very humbly submit that they're derived from establishing a certain tonality. 

I really do hope that somebody with a better or more coherent grasp of common practice period harmony comes along and fixes what I said or amplifies, but I think that's the right idea, anyway.

[keypeg]

Why are both C to Db and C to D considered seconds if the first two are 1 half-step apart and the other two are 1 whole-step apart?

It is best to consider intervals two ways: 1) what they are in reality  2) naming conventions due to written music.

What it is, reallyLet's take the first.  If you play C Db,you are playing a white key, and the black key next to it - the very next key.  They are a "half step" or "semitone" apart.  You hear a particular kind of sound. ---- If you play C C# you will be playing exactly the same piano keys, get the same kind of sound etc. --- The is the reality; what you hear, what piano keys you use etc.  If you think in terms of half steps, whole steps, piano keys touching each other (each a half step apart in sound) then you get like inches or centimeters - a measuring tape "smallest unit" to tell the distance between two tones.

2) naming conventions due to written music.
This is what you are dealing with.  If our C C# is written in the music, both C's will be on the same space or line, one of them having a sharp.  The letter C is followed by the same letter C, and both of them occupy exactly the same position on the staff.  That is the first thing we consider.  It is
a) a unison
b) what kind of unison?

We need to get a handle on a) first

In our C Db, one note is on a space, the other is on the line right above the space, and the letter names are one apart.  C is the 1st note in the sequence C, D, E, F, G, A, B, C --- D is the 2nd note in this sequence.  Since it is the 2nd note in the sequence, It is:
a) a second
b) what kind of second?

For your C D, again we have the 1st note in the sequence, and then the 2nd note, so again, it is

a) a second
b) what kind of second?

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The first thing you must learn to do is recognize whether an interval is a unison, 2nd, 3rd (meaning 1st, 2nd, 3rd, and not the fraction "third" 1/3), 4th, 5th, 6th, 7th, octave.

The second thing is it learn to identify "what kind of" 2nd, 3rd etc.  The "what kind of" is called its quality.

We need that because of how written music has been set up.

Are you able to follow so far?