How are finer distinctions made within the OBAFGKM sequence (e.g., between G0 and G5)?

Answer

By a numerical scale from 0 to 9, where lower numbers are hotter

Each letter designation is subdivided using a numerical scale from 0 to 9; a lower number indicates a hotter star within that specific letter group.

How are finer distinctions made within the OBAFGKM sequence (e.g., between G0 and G5)?
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