Four lines, five characters each, 20 characters total. Rhyme on lines 2 and 4 (and optionally line 1). Li Bai's 《
静夜思》 —
床前明月光,
疑是地上霜。
举头望明月,
低头思故乡 ('Bright moonlight before the bed, I took it for frost on the ground...') — is the form's most-quoted example.