Literally, the code of a piece of track is the height of the rail in thousandths of an inch (meaning code 83 rail is .083" high; code 100 rail is .100" high). The significant difference lies in the physical appearance of the two types of track. Atlas Code 83 track has fine, brown ties whereas Code 100 has slightly thicker black ties. Because of its accuracy (and the color of the ties), Code 83 is more prototypical, and therefore more realistic than Code 100. Code 83 is known as a finer-scale track, and is the choice of discerning modelers. Atlas carries a full line of both Code 100 and Code 83 products.