The period, or full stop, is the first, most important punctuation mark.

The period is used in these cases:

1. at the end of a sentence that is neither a question nor an exclamation:
   
   
   The formation is only 30 m thick at this locality.
   Take out your map.
   
   
   
2. after an abbreviation:
   
   
   Dr.
   Fig.
   ca.
   
   
   In abbreviating the names of organizations, the periods are usually omitted:
   
   
   RCAF
   CMA
   GSC
   IUGS
   
   
   The period that marks an abbreviation is never omitted before a mark of sentence punctuation, except when the abbreviation comes at the end of a sentence. In this situation, the period marking the abbreviation also serves as the period marking the end of the sentence:
   
   
   The firm of Allan and Co., of which I am a partner, has its head office in Ottawa.
   I was made a partner in the firm of Allan and Co.
   
   See also 'Abbreviations' in 'Grammar'.
   
   
   
3. to mark the end of an independent sentence placed inside parentheses. (The period is placed inside the parentheses and the material in parentheses starts with a capital letter and ends with a full stop, as in this example.) If, however, a sentence contains material in parentheses, then the period falls outside the parentheses (as illustrated here).
   
   
   
4. inside quotation marks when the end of the sentence quoted coincides with the end of the main sentence:
   
   
   The excursion guide said, "This unconformity is marked by a regolith, not a shear zone."

The period is not used in these cases:

1. after display lines and titles:
   
   
   Stratigraphy of the Upshot River valley
   
   
   
2. after paragraph headings on separate lines:
   
   
   Detailed stratigraphy of the upper 50 m
   
   
   
3. after box headings in tables:
   
   
   Total organic carbon (%)
   
   
   
4. after date lines and signatures:
   
   
   10 October 1910       Allen J. Moore
   
   
   
5. after SI symbols:
   
   
   All the major events took place within about 15 Ma in that region.
   
   
   
6. after individual letters in acronyms and initialisms:
   
   
   NATO
   UNESCO
   DNAG
   CIMM