When a shoulder strap passes between the breasts. Most often as a result of wearing a bag or purse, but anything with an over-the-shoulder strap (such as a guitar, or some seat belts) can produce this.
As noted in post #648367, in Japan this is referred to as "π/" (pi-slash, "pi" being read as "pai", oppai), or パイスラ (paisura) for short.
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