An electron subshell is a division of a larger electron shell based on energy levels. Energy subshells are denoted by the letters s, p, d, f with each letter indicating a higher energy region. Each subshell corresponds to a particular shape of the location in which electrons can be found, for example the s subshells take the shape of a sphere while the p subshells take the shape of a dumbbell. The number of electrons that can be held in each subshell varies, with s holding 2, p holding 6, d holding 10 and f holding 14. Generally electrons around an atom are primarily divided in energy levels by shell and then subshell, although this is not always the case (for instance the 3d subshell is of a higher energy level than the 4s subshell).