consisting of three letters. Unlike abbreviations, acronyms consist of the first letters of several words. Abbreviations, on the other hand, do not necessarily have to start with the same letters. These two examples illustrate the difference: The acronym TLI stands for Transport LayerInterface, whereas the abbreviation ABX stands for Advanced Branch eXchange
. Whereby the "X" is not the initial letter of Exchange. Just like 3-letter acronyms, there are 2- and 4-letter acronyms.