A JACS code for a single subject consists of a letter and three numbers. The letter represents the broad subject classification, e.g. F for physical sciences. The first number represents the principal subject area, e.g. F3 for physics, and subsequent numbers represent further details, similar to the Dewey Decimal System. The principal subject of physics, for example, is broken into 19 detailed subjects, represented by a letter plus three numbers: e.g., F300 represents physics, F330 environmental physics and F331 atmospheric physics.