Rosalind Franklin used the letter B to denote the hydrated form of DNA, known as B-DNA. In her X-ray diffraction work, she distinguished this form from A-DNA, the less-hydrated form that appears under different moisture conditions.
The distinction mattered because DNA’s diffraction pattern changes with its physical environment. Franklin and her assistant Raymond Gosling studied carefully prepared DNA fibres at King’s College London, controlling humidity and other conditions. The B form produced the famous helical diffraction pattern associated with Photograph 51, taken in 1952.
The X-shaped pattern in Photograph 51 was strong evidence that DNA had a helical structure, although the image alone was not the entire discovery. Franklin’s measurements of spacing, dimensions and water content helped constrain the molecular model later developed by James Watson and Francis Crick. A common misconception is that the letter B was a code for a specific base; it identified a structural form of the DNA molecule, not adenine, cytosine, guanine or thymine.