Shortening and abbreviation An abbreviation

Conventions followed by publications and newspapers

United States

Publications based in the U.S. tend to follow the style guides of The Chicago Manual of Style and the Associated Press. The U.S. Government follows a style guide published by the U.S. Government Printing Office. The National Institute of Standards and Technology sets the style for abbreviations of units.

United Kingdom

Many British publications follow some of these guidelines in abbreviation:

• 
  For the sake of convenience, many British publications, including the BBC and The Guardian, have completely done away with the use of full stops or periods in all abbreviations. These include:
  
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    Social titles, e.g. Ms or Mr (though these would usually have not had full stops—see above) Capt, Prof, etc.;
    
  • 
    Two-letter abbreviations for countries ("US", not "U.S.");
    
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    Abbreviations beyond three letters (full caps for all except initialisms;
    
  • 
    Words seldom abbreviated with lower case letters ("PR", instead of "p.r.", or "pr")
    
  • 
    Names ("FW de Klerk", "GB Whiteley", "Park JS"). A notable exception is The Economist which writes "Mr F. W. de Klerk".
    
  • 
    Scientific units (see Measurement below).
    
• 
  Initialisms are always written in capitals; for example the "British Broadcasting Corporation" is abbreviated to "BBC", never "Bbc". An initialism is also an acronym but is not pronounced as a word.
  

Shortening
Shortening is the process of substracting phonemes and / or morhemes from words and word-groups without changing their lexico-grammatical meaning.
Abbreviation is a process of shortening the result of which is a word made up of the initial letters or syllables of the components of a word-group or a compound word.
Graphical abbreviation is the result of shortening of a word or a word-group only in written speech (for the economy of space and effort in writing), while orally the corresponding full form is used:
days of the week and months, e.g. Sun., Tue., Feb., Oct., Dec.;
states in the USA, e.g. Alas., CA, TX;
forms of address, e.g. Mr., Mrs., Dr.;
scientific degrees, e.g. BA, BSc., MA, MSc., MBA, PhD.;
military ranks, e.g. Col.;
units of measurement, e.g. sec., ft, km.
Latin abbreviations, e.g. p.a., i.e., ibid., a.m., cp., viz.
internet abbreviations, e.g. BTW, FYI, TIA, AFAIK, TWIMC, MWA.