On 29 February 1920 a blue triangle on the hoist was added on previously
used white over red flag, reaching only to one third of the flag. On 30
march 1920 (so one month and one day later!) the triangle was extended
to the centre of the flag (this being virtually the same flag as current
flag of Czech Republic). The reasons I think for adding the blue was to
make it different from the Polish flag which was (re-)adopted about the
same time, and to represent Slovakia (which used white/blue/red tricolour
since 1848). Czechoslovakia consisted of a third part, namely Subcarpathian-Ruthenia.
Mark Sensen, 2 Jul 1996
As I've repeatedly noted, the "1920 flag" ("1/3") is only *proposal*
discussed in the Constituent Assembly, *never* approved, *never* in use.
After negative solution by the parliament, the vexillological commission
returned to this issue and proposed the modified design, with the triangle
reaching the center of a flag. This is the only flag of Czechoslovakia/Czechia
Jan Zrzavy, 31 Mar 2003