甲乙丙丁 (jiǎ-yǐ-bǐng-dīng), the first four Heavenly Stems, are used like A-B-C-D to label contract parties.
甲方 is typically the commissioning/stronger side,
乙方 the service provider — giving rise to Chinese office humor about
乙方的委屈 'the woes of being Party B'.