平 'level' is the old level tone, which became today's first and second tones.
仄 'oblique' covers the other three old tones: rising, departing and entering. The entering tone has vanished from Mandarin and its syllables were scattered across all four modern tones, so some characters that sound first or second tone today, such as
一,
十 and
国, count as
仄 in classical verse.
