Parenthesis means literally 'an insertion beside: something outside the basic meaning of the sentence'. The sentence is logically and grammatically complete without the material contained within the parentheses (round brackets).

Parentheses are used in these cases:

1. to set off words of explanation or comment, or an afterthought:
   
   
   Access to the area is by plane (the landing strips are not always usable), helicopter, or pack horse.
   The study by Hopkins (1949) is the best one completed to date.
   
   A complete sentence that stands alone in parentheses starts with a capital letter and ends with a period:
   
   
   The speaker gave a synopsis of the stratigraphy of the six new formal members. (Detailed measured sections are given in Appendix 5.)
   
   
   
2. to enclose letters or numbers designating items in a series, either at the beginning of a paragraph or within a paragraph:
   
   
   The objectives of this study are 1) to provide vitrinite reflectance profiles, 2) to provide time-temperature histories, and 3) to assess conditions of bitumen reflectance evolution.
   
   
   
3. For the use of parentheses in systematic paleontology, see 'Paleontology'.