妻 (qī) is a bound form (b.f.) — it cannot stand alone as a word in modern Mandarin. It appears as a morpheme in compound words such as 妻子 (qīzi, wife), 夫妻 (fūqī, husband and wife), and 前妻 (qiánqī, ex-wife). Unlike 妻子, which is the common word for 'wife', 妻 is more formal or literary and rarely used alone. Learners should remember to use 妻子 in everyday speech.