一个 (yí gè) is the most frequently used number-plus-measure-word combination in Mandarin. 个 (gè) is the default general measure word used for people and a wide range of objects when no more specific classifier exists. The numeral 一 (yī, one) changes tone to second tone (yí) before a fourth-tone syllable due to tone sandhi.

Beyond simple counting, 一个 often functions as an indefinite article equivalent, introducing a new noun into discourse much like "a/an" in English: 他是一个老师 (He is a teacher). This indefinite-article function makes 一个 one of the most grammatically useful sequences at this level.