This decade is for Earth set between the first two World Wars. During that time, Charleston was a popular dance. The United States economy started to deteriorate, an event which would have consequences even to an alternate timeline (see 1929). During the prohibition era, organized crime rised to never-seen heights. The book Chicago Mobs of the Twenties chronicled this era, and served as a guide for the civilization on Sigma Iotia II.
Scientific breakthroughs on Earth include the launch of the first liquid propellant rocket. Radio became a popular form of entertainment during this decade. In art, the film Metropolis was released and Irving Berlin wrote the song Blue Skies. The song Someone to Watch over Me was also released.