Harmony is typically analyzed as a series of chords. In that hypothetical orchestra, let’s say that the flutist was playing a high A, the violinist bowed a C#, and the trombonist sustained an F#. Together, those three notes comprise an F# minor triad. Therefore, even though each instrumentalist was only playing a single note, together they played an F# minor chord.
When all the instruments in an ensemble are playing notes that fit the same chord, it is known as a consonant chord.
But when players employ a melodic line that does not fit with a set chord (such as an oboist playing a Bb when the rest of the orchestra is playing the tones of a D major triad), it is known as a dissonant chord.
This isn’t to say that some harmonies aren’t intentionally dissonant. Perhaps the composer of that hypothetical piece wanted to hear a Bb over a D chord (in terms of music theory, the note’s harmonic function would be the flat 6th scale degree), even though it wouldn’t be the most pleasing combination to most fans of Western music.
Another common way for composers to express harmony is to declare a particular chord progression and then allow players to craft their own parts to fit that progression.
In the song “Down on the Corner” by Creedence Clearwater Revival:
The song is written in the key of C major.
It uses a common progression of chords for that particular key, mostly fluctuating between a C major triad and a G major triad, with F major triads thrown in at key points.
As such, instrumentalists are expected to use the C major scale to write parts that fit the chord progression.
During the song’s intro section, Stu Cook lays down a bass line that’s mostly single notes, rhythm guitarist Tom Fogerty punches out 5-note and 6-note chords, and lead guitarist John Fogerty plucks a melody based on the C major scale. They are accompanied on drums by Doug Clifford. Everyone is playing in harmony, following both the chord progression and the overall key of C major.