The Allegory of the Cave. The dialogue between Socrates and Glaucon is probably fictitious and composed by Plato; whether or not the allegory originated with Socrates, or if Plato is using his mentor as a stand-in for his own idea, is unclear. In the dialogue, Socrates asks Glaucon to imagine a cave, in which prisoners are kept.
The d sublevel consists of five orbitals that can hold a maximum of  two electrons each. The total maximum number of electrons that the  d sublevel can hold is 10 electrons.
Electron subshells are identified by the letters s, p, d, f, g, h, i, etc., corresponding to the azimuthal quantum numbers (l-values) 0, 1, 2, 3, 4, 5, 6, etc. Each shell can hold up to 2, 6, 10, 14, and 18 electrons respectively, or 2(2l + 1) electrons in each subshell.