krzykat wrote:on the left side of the staff is the key signature, it will show you what notes are sharp or flat (thus tellihn you what scale to use)
and to add to that, when you see a sharp or flat on the staff other than the key signature, your seeing a chromatic note (or a modulation, depends on the context.)
all natural, unmarked notes on the staff (once the key signature is defined) all fall within the diatonic framework... so the key signature is a kind of "set it and forget it" and the notes on the staff become more of a representation of scale degree than identifying a specific note out of context.
So keep playing in that scale, as long as there are no accidentals... once an accidental comes up on the staff you know you've hit an outside-tone.
Is it safe to say that when you read sheet music and it shows the chord that all of the notes in those measure indicated by that chord will be in that scale?
So for example if it's listed as C would you expect all the notes to be in the C major scale?
so to answer your question, it is safe (and practical, and usually right) to assume that all NATURAL notes/chords marked on the staff all belong to the same key of the signature.
but that doesn't mean that "all of the notes in those measure indicated by that chord will be in that scale?" like someone said a cpl posts back, that would be unadventerous boring music.. But if in a measure all of your notes are natural, then they *can* be considered to belong to the same key.
Just remember, everything is contextual, so its just as important to know the song your learning as it is to know how to read sheet music to learn it.
hope that helps