The figures and letters used almost universally to quote Plato refer to a Renaissance edition of his works published in Geneva in 1578 by a famed printer and humanist of the time named Henri Estienne (1528-1598), also known by the Latinized version of his name: Stephanus. This complete edition of Plato's works was in three volumes, whose pages were continuously numbered from the beginning to the end of each volume. Each page of this edition is split into two columns, the right one providing the Greek text and the left one a Latin translation (by Jean de Serres). In between the two columns are printed letters from a to e dividing the column into five sections. 

Based on this, a quotation of Plato includes the name of the dialogue and the page number in the Stephanus edition followed by the letter of the section that includes the first word of the quotation. Thus, quotations take the form Crito 52c or Republic 343d . Quotations are usually given with reference to the start and end point of the quoted section. If the end point is on the same page as the start point, only the end section letter is added, and the quotation takes the form Crito 51 d-e. If the end point is on a different page, the end page number and section letter are provided too, and the quotation takes the form Crito 50e-51b.

(Excerpt from a text by Bernard Suzanne)