A triad is constructed of the first, third and fifth notes of the major scale
G major
1. Understand the Theory
A key signature is the sharp sign next to the clef.
The tonic triad is the 1st , 3rd and 5th note of the scale.
When there is no key signature this triad does not need an accidental as there is no F in the triad to raise by a semitone.
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