Styles differ on whether or not periods should be used with particular abbreviations. The Society discourages the use of periods in certain abbreviations, such as those for academic degrees, names of countries, and other abbreviations that are in all capitals. When periods are removed from abbreviations, the internal space should also be removed (for example, MIT, PhD, US). The internal space is also removed from other abbreviations with internal periods, as with author initials. Note that if an abbreviation also spells a word, removing the following period could cause confusion in some contexts (for example: in., no.).
Although many style manuals decry the use of abbreviations, some words are seldom spelled out. Among these are abbreviations for affiliations or scholarly degrees after a name (BA, PhD), and abbreviations such as AD, CPA, and Ltd. A symbol or figure beginning a sentence, on the other hand, is usually spelled out; if it cannot be, the sentence is rewritten (Two hundred miles, not 200 miles).
Other abbreviation style tips:
The abbreviations Jr., Sr., II and so forth do not require a comma because they are part of the person's name (John Smith Jr.).
Mr., Mrs., and Dr. are dropped if another title is also used (not Dr. John Smith Jr., PhD).
The preferred format avoids using a descriptor unless the company name might not be clear without it, for example, Data Co. However, if it is necessary to use Limited, Incorporated, Corporation, or Company, they are abbreviated to Ltd., Inc., Corp., or Co., respectively, and the abbreviation is not preceded by a comma.
The abbreviations Nat'l and Int'l (for national and international) do not take a period.
The standard abbreviations for most of the degrees we encounter in our authors' biographies are BS, MS, MSc, and PhD. For additional rules on abbreviating academic degrees, see the entry under academic degrees in the alphabetical section.
Centuries use the ordinal symbol, for example, 21st century.
Also see the References section; for a more complete discussion of abbreviations, see CMS, Chapter 10.