An unpronounceable abbreviation comprising the initial letters of a term and commonly used in place of that term. Each letter of an initialism is pronounced independently. Fox example, SDH is the initialism for Synchronous Digital Hierarchy. An acronym is a pronounceable word formed from the initials or other parts of several words that comprise a term. SONET, for example, is the acronym for Synchronous Optical NETwork, the North American standard for fiber optic transmission systems that later became internationalized as SDH. Acronyms and initialisms generally comprise all uppercase letters, although they sometimes are all lower case, or a mixture of upper and lower case. Examples include SONET, SDH, bit (binary digit), bps (bits per second), QoS (Quality of Service), and Sesame (Secure European Systems for Applications in a Multivendor Environment). See also abbreviation, acronym, and contraction.