Band = measured under-bust (rounded to the nearest even inch in modern sizing). Cup = bust − band: 1 inch = A, 2 = B, 3 = C, 4 = D, 5 = DD/E, 6 = DDD/F.
Sister sizes
Same cup volume, different band: go down a band and up a cup (34C ↔ 32D ↔ 36B).
US and UK letters diverge after DD (UK: DD, E, F, FF…; US: DDD/F, G…). EU bands are in cm (75 ≈ 34). Convert with the bra size converter.