The flag of Czechoslovakia belongs to a very small club: flags that outlived their countries. Czechoslovakia stopped existing at midnight on 31 December 1992, and its flag was, by the letter of the dissolution law, supposed to be retired with it. Instead the Czech Republic simply kept flying it, on the argument that a ban written by a dead state binds nobody. Three decades later the white, red and blue wedge design flies over Prague Castle as the Czech national flag, and most people outside the region have no idea it was ever anyone else's.
A Polish problem
When Czechoslovakia declared independence from Austria-Hungary on 28 October 1918, it reached for the obvious flag: white over red, the colors of the Bohemian coat of arms, a white lion on a red field. The trouble arrived within a year. Reborn Poland adopted its own white-over-red flag in 1919, drawn from its own white eagle on red, and the two new neighbors found themselves flying practically the same banner at each other across a contested border. Given that Poland and Czechoslovakia spent 1919-1920 in an actual territorial dispute over Teschen, the ambiguity was more than an aesthetic problem.
A commission went to work on alternatives, and the winning design came from inside the bureaucracy: Jaroslav Kursa, an archivist in the Ministry of the Interior, kept the Bohemian white and red and drove a blue isosceles triangle in from the hoist. The National Assembly approved the flag on 30 March 1920. The wedge reaches the flag's midpoint, the proportions are 2:3, and the design has not changed since.
The triangle did two jobs at once. It made the flag unmistakable at any distance from Poland's, and it completed the pan-Slavic color set of white, blue and red. The blue was widely read as Slovakia's presence in the flag, joining Bohemian white and red; later readings also point to the blue in Moravia's arms. For a state assembled from pieces of a collapsed empire, a flag whose geometry visibly joined its parts was good politics. It was also, quietly, unusual: hoist triangles were rare in 1920, when Cuba and the Philippines were among the few flags using one, and none drove the wedge as deep as Kursa did.
Banned, restored, retained
The flag's first death was imposed from outside. After the German occupation of March 1939, the Protectorate of Bohemia and Moravia banned the Czechoslovak flag and substituted a horizontal tricolor of white, red and blue; the nominally independent Slovak State flew a Slovak tricolor with a double cross. The Czechoslovak government-in-exile in London kept the 1920 flag alive on official documents and Free Czechoslovak units fought under it. It came home with the restored republic in May 1945.
The communist coup of February 1948 changed the coat of arms, the constitution and nearly everything else, but never touched the flag. It flew through the Prague Spring and its crushing in August 1968, when photographs showed Czechs carrying the flag past Soviet tanks in the streets, and it was everywhere on Wenceslas Square during the Velvet Revolution of November 1989. By the time the country approached its second death, the flag had accumulated three quarters of a century of shared history.
The clause both sides signed and one side ignored
The Velvet Divorce was negotiated, not fought, and its legal instrument was Constitutional Act 542/1992 on the dissolution of the Czech and Slovak Federative Republic. Tucked into it was a clause, Article 3, forbidding both successor states from using the state symbols of the federation: flag, arms, anthem and the rest. The intent was symmetrical fairness. Slovakia had already adopted its own flag in September 1992, a white-blue-red tricolor bearing the Slovak double-cross arms, so the clause effectively asked the Czechs to invent a new flag from scratch in about three months.
They tried, briefly. Proposals circulated in late 1992, but none had any support compared to the flag everyone already regarded as their own; Slovak identity had its tricolor, and Czech identity had, in practice, the federal flag. So on 17 December 1992 the Czech National Council passed Act 3/1993 on the state symbols of the Czech Republic, adopting the 1920 flag wholesale, effective the day the new state came into being.
Slovak politicians objected that this violated the dissolution act, and they were right on the text. The Czech answer was procedural and unanswerable: the act was a law of Czechoslovakia, Czechoslovakia no longer existed, and no court or body remained with jurisdiction to enforce its terms against a sovereign successor. Slovakia registered its displeasure and moved on. No sanction, suit or serious diplomatic rupture followed, and within a few years the matter was a footnote. The blue wedge, originally added partly to make room for Slovakia, now flies over a state with no Slovaks in its name.
The survivor
As a piece of design, the flag has aged remarkably well: three colors, no lettering, one strong geometric idea, distinguishable at a glance from every neighbor. As a piece of history, it is stranger. It is simultaneously a 1920 solution to a Polish lookalike problem, a banner banned by one occupation and paraded against another, and the subject of a constitutional ban that lasted exactly as long as the country that enacted it. The Czech Republic celebrated the flag's centenary in 2020, one hundred years after Kursa's wedge was approved, and 27 of those years belonged to a country the flag was never designed for.
References
[1] "Flag of the Czech Republic," Wikipedia, covering Kursa's 1920 design, the 30 March 1920 adoption, Act 3/1993 and the dissolution controversy. https://en.wikipedia.org/wiki/Flag_of_the_Czech_Republic
[2] Government of the Czech Republic, "The Czech Republic's national flag," official history page. https://vlada.gov.cz/en/clenove-vlady/historie-minulych-vlad/historie-statni-vlajky/the-czech-republics-national-flag-19198/
[3] Constitutional Act No. 542/1992 Coll., on the dissolution of the Czech and Slovak Federative Republic, including the clause on state symbols.
[4] Act No. 3/1993 Coll., of the Czech National Council, on the state symbols of the Czech Republic, 17 December 1992.
[5] Flags of the World (FOTW), "Czech Republic / Czechia," on the 1993 retention of the federal flag and Slovakia's objection. https://www.fotw.info/flags/cz.html