Flag: Czechia

It stands for Czechia, the Czech Republic, and anything Czech.

The flag has a white band on top, a red band below and a blue isosceles triangle starting at the pole edge and reaching the centre of the flag. In messages it does what any country flag does: it marks a place, a team or an identity. The most common sightings are travel posts from Prague, Český Krumlov or a Czech ski resort, scores and line-ups during ice hockey tournaments, where Czech fans are famously loud (the national team won Olympic gold at Nagano in 1998, still a national memory), and bios where someone signals Czech nationality or Czech ancestry. Czech beer is the other big trigger: pilsner-style lager was born in Plzeň in 1842, and the original Budweiser comes from České Budějovice, so 🇨🇿 often sits next to 🍺.

On the Czech calendar the flag spikes on 28 October, the holiday marking the founding of the Czechoslovak state in 1918, and on 17 November, which commemorates the 1989 Velvet Revolution.

Why is it the same as the Czechoslovak flag?

Because Czechia simply kept it after the 1993 split.

White and red were the historic colours of Bohemia, and they formed the first flag of Czechoslovakia in 1918. Since Poland flew the same two bands, a blue triangle was added in 1920, in a design by the archivist Jaroslav Kursa; blue was read as standing for Moravia and Slovakia. When Czechoslovakia divided peacefully into Czechia and Slovakia on 1 January 1993, a 1992 federal law said neither new state should use the old federal symbols. Czechia adopted the flag anyway, Slovakia designed a new one 🇸🇰, and the matter faded. So if someone uses 🇨🇿 for Czechoslovakia in a history post, it is not wrong-looking, but today the emoji means Czechia.