to understand something very clearly, as if pointing to things in one's own palm; to know something inside out

to know something or someone extremely well, as if looking at things on the palm of one's hand; to know like the back of one's hand

An important 12th-century rhyme table traditionally attributed to the Song Dynasty scholar [[司马光|司馬光|si1 ma3 guang1]] (Sima Guang), used for studying historical Chinese phonology; it organizes the sounds of Chinese into twenty charts based on the thirty-six initials.

clear and easy to understand as if pointing at one's own palm