The shelf bra, also called a platform bra, is characterized by a multipiece cup mounted above an elastic facing and cup frame (BM4110). This concept also applies to the shelf halter, which is also discussed here (SS4130, MS4310). This species is also called a platform brassière. (And it is related to the sister species, the platform halter; see the soft-top halter for a few examples.)
The shelf plays a major development role in the early 1940s as the belly opens up (SS4110). The shelf provides coverage below the breasts as well as foundation. Technologically, the circumference of fabric around the chest constrains the midriff exposure from crawling up and impinging the breasts.
Because the shelf bra (or halter) has a strong horizontal foundation component it works well with culotte that also have strong horizontal elements. When worn with shorts it composes more of a playsuit than a swimsuit (US5002), but here especially a very high waistline can combine with the shelf to compress midriff (SS4140). These include wide waistbands, belts, and of especially the sheath in which an excess of elasticized fabric protects the front of the bottom (SS4130).
As the 1940s progress the bare midriff widens, and the importance of the shelf bra wanes. Cleavage remains de rigeur, foundation continues to play a role, but the amount of fabric in the bra recedes, and the bra looses its shelf, sometimes little by little (LM48G5). The species remains popular well into the 1950s, if only because the platform can aid in foundation for cleavage (DO5810). The elimation of the platform gives rise to the normal brassière.
The shelf bra is identified by a platform of support below the bra (or halter) cups.