码
mǎ · HSK 7
number
石 stone + 马 mǎ (horse, lends the sound) = 码; a measure word, mǎ like 马
The 石 stone radical hints at counting solid units; 马 mǎ just carries the pronunciation. As a classifier 码 tags matters or yards, e.g. 两码事 = two separate things.
码 — stone radical 石 + 马 (mǎ, horse, phonetic)
码 is a semantic-phonetic compound. The stone radical 石 hints at the original meaning: a small stone or weight used for counting or measuring. 马 supplies the sound mǎ. Over time the meaning broadened to 'numeral, code, yard (unit),' as in 号码 (number) and 密码 (password).