Using chords with notes that doesn't belong to the scale

[dark_virus]

Didn't read your post. Thanks for these tips, ecsmix.

[djriddle]

The main point is: since Gb and G# aren't part of the C major scale, these notes have a good chance of clashing with others, right? So, is there some tip on how to use them or is it better to avoid them?

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There are no notes you should "avoid". As a piano teacher, I always think it's funny when kids (and adults, too) decide some keys on the piano are "scary" or "hard" and shouldn't ever be played. The best way to learn is just to listen...listen, and see what you like.