I'm expecting responses from the composition principles. I'm asking for a rule that explains when and why to use thirds in melody. (sorry for my english) I will visualize this problem on two diffrent melodies. First *1 I can't imagine without thirds, but the second *2 I can't imagine with thirds. so, there is any empirical rules on the use of thirds? *1
When doubling a melody in thirds, the added note should complete the RIGHT chord, on the strong beats at least. Sometimes a third above works well, sometimes a third below. Sometimes neither is appropriate. (If you want to retain the melody line on top, note that a third above is harmonically equivalent to a 6th below.)
As with so many musical matters, it can be easier to point out situations where it DOESN'T work well than to create rules about when it DOES.
The only time that you need to follow rules is when you are trying to duplicate a particular style or genre. The most obvious examples of this are the exercises in exams where you are asked to harmonize a tune in the "style of a Bach Chorale".
Apart from that you are free to do whatever you want to. It is your composition and it should sound how you would like it to sound. It does not have to conform to anything unless you want it to.
Having said that there are some musical forms that are often followed by composers. For example Sonata form and Rondo are two forms that give structure to a piece and are often used by composers. But there are no rules; you are free to use then if you wish and you are also free to ignore them if it is not what you, as the composer, wish to do.
Hope that helps. Good luck with your writing!
The use of 'chordal melodies' if you will, is much more of a specific feature of the piano. Both these pieces, seem to be keyboard music of some sort and the thing with a piano is that the notes have a very short decay unless you hold the note, it is not going to ring.
This combined with the fact that the piano as a base had a great appeal because of its increased sonority, you really need to work hard to play guitar to a great big concert hall, but a piano does it much more naturally.
These use of thirds and octaves are not done from a purely harmonic or melodic perspective, they are done because piano needs a lot of notes and often the playing of octaves are just to make it sound more impressive / better.
These are stylistically relevant not harmonic/melodic.
If you want to hear a bare melody on the piano the one that comes to mind is the girl with the flaxen hair by Claude Debussy.