Fit Problem: Quad or double-boob line

The short answer: The cup edge cuts across tissue because the cup is too small or the style too low-cut. Go up one cup letter. If it only happens in one brand, that brand's cups run shallow for you.

The quad effect, tissue shelving over the cup's top edge to create four contours instead of two, is top-edge spillage with a memorable name. Under a t-shirt it draws a hard line exactly where clothing should skim, which is why it is the fit problem people notice in photos before they notice it in mirrors.

The default fix is one cup letter up at your band: quad lines mean the cup edge landed on tissue instead of past it. But style participates too: low demi and plunge edges cross the tissue of anyone with upper fullness, and stiff foam edges draw lines where soft-fabric edges blend. If your letter is verified and the quad persists in low cuts only, the fix is coverage height or a stretch-lace edge, not more letters.

Timing matters as well: sizes that quad only premenstrually or mid-cycle are telling you breast volume genuinely fluctuates, which it does for most people. A sister size up kept for those weeks costs less than wearing lines through half of every month.