Commas are a very necessary form of punctuation. Missing them out, using them wrongly or over-using them makes text hard to understand; when commas are missed out, ambiguities creep in; when wrongly used, they disrupt a sentence or phrase; when over-used, they not only disrupt the sentence, but result in 'run-on' sentences, i.e. sentences where commas are used instead of full stops, creating one sentence from what should be three or four sentences.
Here is an example of a run-on sentence:
- The Athenian empire effectively used public space in Athens to create and promote political identity, the Agora provided an open space for Athenians to converse openly about the issues discussed in the Acropolis, the Acropolis enhanced as well as created Athenian identity.
The sentence should be divided up:
- The Athenian empire effectively used public space in Athens to create and promote political identity. The Agora provided an open space for Athenians to converse openly about the issues discussed in the Acropolis; the Acropolis enhanced as well as created Athenian identity.
Here are some examples of good comma use.
separating items in a list:
- He demonstrates valour, clemency, justice and piety.
N.B. If the list is long and complicated, it's sometimes also helpful to put a comma before the last item. This is known as a serial comma.
- He demonstrates valour of the warrior, the clemency of the powerful, the justice of the sage and the piety of the priest, and is still seen to be human.
joining two related sentences, the second of which begins with and, or, but, yet or while:
- The Agora was a market place, but the building of the law courts changed its nature.
marking the beginning and end of a sub-clause - a subclause is a phrase that provides additional information:
Octavian assumed a new name, Augustus, after winning the battle of Actium.
Prometheus, who remains on stage throughout, provides a fixed point.
introducing or within quoted speech:
- One such moment is when Aeneas, carrying his dying father from burning Troy, asks 'how could a father say it?' (cite reference)
- 'Did you suppose, my father,' asks Aeneas, 'that I could tear myself away and leave you?' (cite reference)
If you put commas in the wrong places, or miss them out when they are needed, your work will be hard to understand. This will cost you marks. If you are unsure about a comma, read your work out loud. If you need a pause, you need a comma.
A useful tip: the more good writing you read, the better attuned your ear will become. Eventually, comma placement will be instinctive.