Also known as IMF, or Infra Mammary Ridge or Underboob fold.

The Inframammary fold refers to the crease below the breast that delimits the breast from the chest wall.

Bigger breasts will start folding at this point and skin to skin contact may start to happen.

On smaller breasts, this section refers to the bottom end of the breast tissue on the ribcage.

Relationship with bra fit

Your bra's underwire should sit on your IMF. If your bra is sitting below your IMF it could mean a few different things but overall the bra is not properly worn and most likely not right for you.