Historically, the ionosphere was thought to be composed of a number of relatively distinct layers that were identified by the letters D, E, and F.
The F layer was subsequently divided into regions F1 and F2. It is now known that all these layers are not particularly distinct, but the original naming scheme persists. Using the symbol E to describe the electric field of the wave reflected from the first layer of the ionosphere and the second layer at higher altitude and used the symbol F for the reflected wave and the lower altitude region is denoted as the symbol of D. In time, the letters came to be associated with the layers themselves rather than with the field of the reflected waves. It is now known that electron density increases more or less uniformly with altitude from the D region, reaching a maximum in the F2 region.