When the outer side of the chest is visible through a gap in the clothing being worn by the subject, or if only the side of the chest is visible with, for example from behind angles (Danbooru post #4805923, Danbooru post #6728910, Danbooru post #5224430).
If the nipples are visible, use nipple slip as well (Danbooru post #4343645).
Do not confuse with pectoral cleavage.
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