Cutoff line and waistline.
Shifting erogenous zones are illustrated in this Belly-Up / Belly-Down diagram, which tracks midriff exposure between the late 1930s and the year 2000 (1). The initial date marks the introduction of the maillot cutout, represented by the triangle on the graph, and the first deux-pièces. Belly-up exposures are bounded by breast cleavage underside (cleavge neathage), and belly-down exposures are bounded by waistline. Hairage and rugage define the maximum depth of belly-down.
Exposures of midriff may or may not involve navel; if they don't, they are confined to either above it--the classic 1940s position--or below it, a rather position seen occasionally during the late 1970s and more frequently thereafter. In all other cases, that is when the belly button is present, midriff has relationship between two separate vectors, one up the ribs, one down the pelvis.
After the belly button is freed in the late 1940s and early 1950s, the dominant trend is belly-down; in the 1980, the midriff is closed in from both directions, but it will remain playfully exposed with all combinations running wild.
More on the theory of shifting erogenous zones is found at the Garden of Eden.
Judson Rosebush, Second Skin Manuscript, 1988. 1 picture.