https://www.marxists.org/archive/marx/works/1848/communist-manifesto/preface.htm

People don't read prefaces enough. It's fully of juicy deets:

Nevertheless, when it appeared [The Communist Manifesto], we could not have called it a socialist manifesto. In 1847, two kinds of people were considered socialists. On the one hand were the adherents of the various utopian systems, notably the Owenites in England and the Fourierists in France, both of whom, at that date, had already dwindled to mere sects gradually dying out. On the other, the manifold types of social quacks who wanted to eliminate social abuses through their various universal panaceas and all kinds of patch-work, without hurting capital and profit in the least. In both cases, people who stood outside the labor movement and who looked for support rather to the “educated” classes. The section of the working class, however, which demanded a radical reconstruction of society, convinced that mere political revolutions were not enough, then called itself Communist. It was still a rough-hewn, only instinctive and frequently somewhat crude communism. Yet, it was powerful enough to bring into being two systems of utopian communism — in France, the “Icarian” communists of Cabet, and in Germany that of Weitling. Socialism in 1847 signified a bourgeois movement, communism a working-class movement. Socialism was, on the Continent at least, quite respectable, whereas communism was the very opposite. And since we were very decidedly of the opinion as early as then that “the emancipation of the workers must be the task of the working class itself,” [from the General Rules of the International] we could have no hesitation as to which of the two names we should choose. Nor has it ever occurred to us to repudiate it.

I think it's interesting how words change meaning. How Lenin and Stalin both treated Communism and Social Democracy as synonymous in the 1900s, but by the mid 1920s Social Democracy was regarded openly as the moderate wing of fascism, by Stalin. It makes me wonder how much confusion this causes.