A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007
How to format abbreviations
Abbreviations
Part III. Style 20 spelling
Although abbreviations follow the general principles discussed here, there are many exceptions.
Capitalization. Abbreviations are given in all capital letters, all lowercase letters, or a combination.
BC
p.
Gov.
CEO
a.m.
Dist. Atty.
U.S.
kg
PhD
Punctuation. In general, abbreviations given in all capital letters do not include periods, while those given in lowercase or a combination of capital and lowercase letters have a period after each abbreviated element. However, as you can see from the examples above, there are exceptions: the abbreviation U.S. includes periods (see 24.3.1); metric units of measure (see 24.5) are in lowercase without periods; and the abbreviation PhD has no periods. Other exceptions are noted throughout this chapter.
Spacing. In general, do not leave a space between letters in acronyms (NATO) and initialisms (PBS), but do leave a space between elements in abbreviations formed through shortening (Dist. Atty.). If an abbreviation contains an ampersand (&), do not leave spaces around it (Texas A&M). For spaces in personal names, see 24.2.1.
Typography. Do not italicize abbreviations.
Indefinite articles. When an abbreviation follows an indefinite article, choose between a and an depending on how the abbreviation is read aloud. Acronyms (NATO, AIDS) are pronounced as words; initialisms (EU) are read as a series of letters.
member nation of NATO
a NATO member
person with AIDS
an AIDS patient
member nation of the EU
an EU member