An anarchist flag is a slight contradiction. Flags mark nations, armies, and parties, all things anarchists have spent 150 years refusing. The movement resolved the problem in character: it adopted the anti-flag. A plain black rectangle, the absence of every national colour, has been the banner of anarchism since the 1880s, and a red-and-black field split on the diagonal has been the banner of its trade-union wing since the 1930s. Neither was ever adopted by any congress or written into any constitution. They spread the way anarchist ideas spread, sideways, by example.
Out from under the red flag
For most of the nineteenth century, anarchists marched under red. They were part of the broad socialist movement and shared its banner, its songs, and for a while its First International, until Marx and Bakunin's factions split it in 1872. As the anarchists became a distinct movement, they needed to look distinct, and black gradually took on the job.
The associations accumulated through the late 1870s and early 1880s. A London-based anarchist congress of 1881 launched what was nicknamed the Black International. In Spain, authorities blamed a shadowy (and possibly invented) society called La Mano Negra, the Black Hand, for peasant unrest in Andalusia. And in Paris, the veterans of the crushed Commune of 1871 carried a decade of mourning with them. Black was the colour of hunger, of funerals, and of refusal.
Louise Michel's broom handle
The flag has an unusually precise birthday. Louise Michel, schoolteacher, Commune veteran, and by then the most famous anarchist woman in France, had argued at a Paris meeting on March 18, 1882 that anarchists should drop the red flag for the black. On March 9, 1883, she put it into practice, leading a demonstration of unemployed workers at the Esplanade des Invalides in Paris behind a black flag improvised from a rag and a broom handle. By her account it was a flag of mourning and of strikes, the sign that the worker had no bread. The demonstration turned into the looting of three bakeries, and Michel was sentenced to six years in prison, largely for carrying the flag at the head of the crowd.
The symbol outran the sentence. A French anarchist paper called Le Drapeau Noir, The Black Flag, began publishing in August 1883. By November 1884 black flags were appearing at anarchist demonstrations in Chicago, carried by the milieu around Albert and Lucy Parsons two years before Haymarket. From there the flag simply became standard. When Nestor Makhno's Revolutionary Insurgent Army fought across southern Ukraine between 1918 and 1921, defending peasant self-government against Whites and Bolsheviks alike, its columns rode under black banners, some embroidered with "Liberty or Death." Anarchist publications ever since have reached for the same name: Black Flag in Britain, Black Flag in America, and dozens of equivalents elsewhere.
The plain black flag means, roughly, everything the national flags are not. No device, no motto, no colour. Anarchists have glossed it as the negation of borders and states, as mourning for the movement's dead, and as stubbornness itself. Howard Ehrlich's much-reprinted essay "Why the Black Flag?" reads it as the negation of all flags. No definition is binding, because no body exists that could bind one.
Red and black together
The second flag stitches the black back onto the red it left. Red and black had appeared together in anarchist hands early: Italian anarchists wore red-and-black cockades in the Bologna insurrection of 1874 and carried a red-and-black flag into the town of Letino during the 1877 Benevento rising. But the diagonal bisected flag, one triangle red and one black, is Spanish, and belongs to anarcho-syndicalism: the strategy of organizing revolution through mass trade unions.
Spain had the largest anarchist movement in the world. The CNT, the National Confederation of Labour, founded in Barcelona in 1910, counted over a million members at its height; the FAI, the Iberian Anarchist Federation, founded in 1927, was the specifically anarchist organization within it. The red of the syndicalist unions and the black of the anarchists met in one flag, split on the diagonal so that neither colour sat above the other by design. Vexillological sources generally place its first public display at the CNT-FAI's May Day demonstration in Barcelona in 1931, weeks after the proclamation of the Second Republic.
The flag's great moment came in 1936. When the generals rose against the Republic in July, CNT militias helped defeat them in the streets of Barcelona, and for about a year much of Catalonia and Aragon ran on anarchist lines: collectivized factories, peasant communes, militia columns with names like Durruti. The red-and-black flew over all of it, on armbands, bandanas, armoured cars, and requisitioned hotels. George Orwell, who fought in Catalonia with the allied POUM militia, described revolutionary Barcelona in Homage to Catalonia as a city where every building of any size had been seized and draped with red flags or the red and black flag of the anarchists. Defeat in 1939 scattered the CNT into exile and clandestinity, but the flag survived as the standard emblem of anarcho-syndicalist unions: the revived CNT and the CGT in Spain, the IWA federation internationally, and the red-and-black strike flags still hung on Mexican workplaces to announce a huelga.
Two flags, one refusal
The two flags divide the labour between them. Plain black is the broadest statement, anarchism as such, and turns up wherever the movement does, from squats to summit protests. The red-and-black diagonal claims the syndicalist inheritance specifically, and later movements have run their own colours through the same template: green over black for green anarchism, purple over black for anarcha-feminism, black and yellow variants for others. The design language is open source, like the politics. What the variants share is the black triangle and the diagonal cut, and the absence of anyone with the authority to say they are doing it wrong.
References
[1] "Anarchist symbolism," Wikipedia. https://en.wikipedia.org/wiki/Anarchist_symbolism
[2] "Demonstration of 9 March 1883," Wikipedia. https://en.wikipedia.org/wiki/Demonstration_of_9_March_1883
[3] Flags of the World (CRW Flags), "Anarchism and the origin of the black flag." https://www.crwflags.com/fotw/flags/fr%7Danar.html
[4] Flags of the World (CRW Flags), "Anarcho-syndicalism: the diagonal red and black flag." https://www.crwflags.com/fotw/flags/qt-a_dia.html
[5] "Red and black flag," Wikipedia. https://en.wikipedia.org/wiki/Red_and_black_flag
[6] Louise Michel, The Red Virgin: Memoirs of Louise Michel (University of Alabama Press, 1981).
[7] George Orwell, Homage to Catalonia (1938), on revolutionary Barcelona under CNT-FAI flags.
[8] Howard J. Ehrlich, "Why the Black Flag?", in Reinventing Anarchy, Again (AK Press, 1996).