For each of the eighty-eight officially designated constellations in the sky, there is a corresponding standard TLA, or Three-Letter Abbreviation. These abbreviations are used in place of the full constellation name for convenience, where space is limited, or to keep data a consistent length (as for example in a database).
In general, a constellation's abbreviation is simply the first three letters of its name (or in the cases of Ara and Leo, the constellation's entire name). There are, however, many cases where this simple approach would lead to ambiguity or confusion, and in these cases more specific abbreviations are used. For example, there are four constellations whose names begin with the letters 'Can...' (Cancer, Canes Venatici, Canis Major and Canis Minor). In cases likes these, abbreviations are formed from constituent letters within the name - so in this case the abbreviations are 'Cnc', 'CVn', 'CMa' and 'CMi' respectively.
Abbreviations can be used as simple alternatives to the full constellation name (so for example 'Cen' for 'Centaurus'). They can also be used in place of genitive forms in stellar designations, so Alpha Centauri would simply be 'Alpha Cen'. Often in these cases the Greek Bayer designation will also be abbreviated to three letters, in this case producing 'Alf Cen' ('Alf' being the conventional contraction of 'Alpha').* Similarly the abbreviation can be used in other systems using genitives, so for example the Flamsteed designation 51 Pegasi is commonly seen in the shortened form '51 Peg'.
The table below shows the eighty-eight standard abbreviations in their lowercase form, in order to show the conventional capitalisation. It is very common to see these abbreviations in block capitals (e.g. 'CMA' for Canis Major, rather than 'CMa'). Since all eighty-eight trigraphs are distinct from one another, there is no risk of confusion regardless of the capitalisation scheme used.