Franklin and Gosling's Photo 51 showed the B-DNA form of DNA.
Photo 51 was an X-ray fibre-diffraction image made from a highly hydrated DNA sample. Raymond Gosling produced it while working with Rosalind Franklin at King's College London; the image was the 51st diffraction photograph in his series, which explains its name.
B-DNA is the familiar, relatively hydrated form of the DNA double helix found under ordinary cellular conditions. Its distinctive X-shaped diffraction pattern indicated a helical structure. The spacing of the spots also helped researchers estimate important dimensions of the helix.
The image did not, by itself, reveal every detail of DNA's molecular structure. Watson and Crick combined its evidence with other experimental results, including Franklin's analyses and work by Maurice Wilkins. A common mix-up is A-DNA: Franklin studied both forms and was especially focused on A-DNA at one stage, but Photo 51 is associated with B-DNA.