A parenthesis is additional information added into a sentence as an explanation or an afterthought. A parenthesis can be shown using two brackets, two commas, or two dashes. The choice is yours. Here are some guidelines:
- Using Dashes: Your parenthesis will be easily seen, but dashes can look a little stark.
- Using Commas: Commas make for a normal-looking sentence, but commas can be confused with other commas in the sentence.
- Using Brackets: Your parenthesis will be easily seen, but brackets can make official letters look a little unorganised.
For Parenthesis, Use Commas, Dashes or Brackets:
A parenthesis is additional information added into a sentence as an explanation or an afterthought. For example (parenthesis shaded):
- John, a 7-year-old cat from Doncaster, hid in the engine area of his owner's car for a 60-mile trip to the seaside.
- Peter, I've heard it said, used to be a drag queen before he took up body building.
When a parenthesis is completely removed, the sentence is still grammatically correct. A parenthesis can be separated from the rest of the sentence by commas, dashes, or brackets (all called parentheses).
(Note: The word parentheses is often used to mean brackets. Therefore, to avoid confusion, you will often see brackets (i.e., parentheses), commas, dashes grouped under the term parenthetical punctuation as opposed to parentheses.)
It is your choice which style of parentheses you use. It is normal to use commas, but they can be easily confused with other commas in the sentence. Brackets will make your parenthesis easily identifiable, but brackets can look a little informal. To make your parenthesis really stand out, you can use dashes, but they can look a little stark.
Try reading each example with the parenthesis removed. It will still make sense. 
- Jamie Buxton, who fainted in church during his wedding, apologised to his wife by booking two tickets to New York. (The parentheses chosen by the writer were commas. However, brackets or dashes could equally have been used.)
- At midnight last night, Skip (a guard dog for Bonds Ltd in Bury) hospitalised two burglars before returning to eat the steaks they had thrown him. (The writer has chosen brackets because there is already a comma in the sentence.)
- Dave Jenkins' best friend, Adam Wright-Smith, stabbed him through the heart whilst testing a knife-proof jacket; Dave is expected to make a full recovery. (The writer has chosen commas, possibly because there are already two hyphens in the sentence, and dashes look similar to hyphens.)
However, As a Result
Inserted comments such as however, therefore, as a result, as far as I am concerned, for all intents and purposes, subsequently, so to speak, etc. fall into the category of parenthesis too. (As a rule, brackets are not used with these.)
Examples:
- The slow cooker I purchased at your store is, for all intents and purposes, utterly useless.
- Darius, on the other hand, writes his own songs.
- It rained all day and, as a result, the hut collapsed.