Why does the H-R diagram place O-type stars on the far left side?

The scale preserves the historical spectral classification order.

The placement of spectral types on the horizontal axis of the H-R diagram reflects historical observational categorization rather than a simple numerical progression of temperature. Hottest stars, classified as O types, are placed on the far left because this aligns with the original sequence established by early astronomers, such as Ejnar Hertzsprung and Henry Norris Russell, when they first plotted their data. Although O types are the hottest (exceeding 30,000 Kelvin), the convention dictates that moving to the right progresses through cooler types like G (the Sun) down to M types (below 3,500 Kelvin). If the axis were ordered purely by ascending temperature from left to right, the entire diagram's layout would be horizontally inverted.