Guys guys guys we really need to make this one thing clear
One of the main differences is that dandyism was also an intellectual movement that stressed self-improvement in order to become a living work of art; later in the 19th century, it was closely associated with the Aesthetic, Decadent, and Symbolist Movements. Foppery never had any intellectual underpinnings, but because dandies were considered flamboyant by 19th century standards, they were often perceived as fops anyway.
"We, who have different interests, never would we have thought, even in our dreams, that one day, we would wear the same jewel. You, who likes new things, who tends to fall for too many guys too quickly… I hope that even today you’re still treasuring and wearing that old shop’s standard ring!"
(Source: hojo-kun, via josephinepoe)