Conversation so far:
What is progressiveness?
Progressive rock (or art rock) grew out of the classically-minded strains of British psychedelia.
What is psychedelia?
fused elements of cultivated music with the vernacular traditions of rock.
How did the Beatles contribute to progressiveness?
is generally credited as sparking a musical craze for the sound of the instrument in the mid-1960s.
What instrument did the Beatles spark a musical craze for?
droning guitars.
When did the Beatles start using droning guitars?
a trend which would later be associated with the growth of raga rock, Indian rock, and the essence of psychedelic rock.
Anything else interesting in the article?
For several years following Sgt.
Pepper's release, straightforward rock and roll was supplanted by a growing interest in extended form.

Question: When was Sgt. Pepper's released?
Self-contained rewrite: When was Sgt. Pepper's released?