What do the numbers and letters mean on interchangeable lenses?

Basic lens specifications are inscribed on interchangeable lenses. The numbers and letters provide the focal length, maximum aperture (f-stop setting), filter size, and other information related to the lens.

For example, on the front of the SAL1870 lens the inscription reads: DT3.5-5.6/18-70 and 0.38m/1.3ft MACRO and Ø55. The DT prefix indicates that the lens is specifically designed for use with APS-C format digital SLR cameras. The 3.5-5.6 numbers indicate the maximum aperture (f-stop setting) possible at the shortest and longest focal lengths. The 18-70 indicates 18mm is the shortest focal length and 70mm is the longest focal length. The 0.38m/1.3ft designation indicates the minimum focusing distance of the lens in meters and feet. The Ø55 indicates the size (in millimeters) of filter you can attach to the lens.