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Initially Tin Pan Alley specialized in melodramatic ballads and comic novelty songs, but it embraced the newly popular styles of the cakewalk and ragtime music. Later, jazz and blues were incorporated, although less completely, as Tin Pan Alley’s primary orientation was producing songs that amateur singers or small town bands could perform from printed music. Since improvisation, blue notes, and other characteristics of jazz and blues could not be easily captured in conventional printed notation, Tin Pan Alley manufactured jazzy and bluesy pop-songs and dance numbers. Much of the public in the late 1910s and the 1920s did not know the difference between these commercial products and authentic jazz and blues.

from https://acousticmusic.org/research/history/musical-styles-and-venues-in-america/tin-pan-alley/

Jazz Harmony: a useful overview

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In Britain?