The form of DNA that is shorter and wider than B-DNA and occurs at low humidity is A-DNA.
A-DNA is a right-handed double helix with a more compact shape than the familiar B-DNA form. It has about 11 base pairs per turn, compared with roughly 10.5 in B-DNA, and its base pairs are tilted relative to the helix axis. This gives A-DNA a shorter, broader appearance.
B-DNA is the dominant form under the watery conditions found inside cells. When DNA loses water, such as under low-humidity conditions, its structure can shift toward A-DNA. A-DNA-like geometry is also found in DNA–RNA hybrids and double-stranded RNA, because the ribose sugar in RNA favors this shape.
The letter does not indicate a different genetic code: A-DNA, B-DNA and Z-DNA contain the same bases but differ in three-dimensional geometry. Z-DNA is especially easy to confuse with A-DNA because it is another alternative helix, but Z-DNA is left-handed and has a zigzagging backbone.