A triad is constructed of the first, third and fifth notes of the major scale
C sharp major
1. Understand the Theory
A key signature is the sharp signs next to the clef. In this case we have F sharp, C sharp, G sharp, D sharp, A sharp, E sharp & B sharp (played as the note C) . If you need help here please see the key signatures blog.
The tonic triad is the 1st , 3rd and 5th note of the scale.
When there is no key signature this triad requires an sharp sign in front of the C, E and G to make an C sharp, E sharp and a G sharp.
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