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Narrow, shoulder-width dips primarily train the triceps, with major synergists being the anterior deltoid, the pectoralis muscles (sternal, clavicular, and minor), and the rhomboid muscles of the back (in that order). Wide arm training places additional emphasis on the pectoral muscles.
The exercise works the pectoralis major as well as supporting chest, arm, and shoulder muscles such as the anterior deltoids, serratus anterior, coracobrachialis, scapulae fixers, trapezii, and the triceps.
Motion developed: bench press.