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.

A run-on sentence might look like this: A significant risk attached to digital databases is false identification, a common feature of personal blogs is displaying a face on your profile that is not actually your own, celebrity lookalikes are very popular, therefore extending the police database to include personal profiles will result in high levels of false allegations.

This sentence should be divided up: A significant risk attached to digital databases is false identification. A common feature of personal blogs is displaying a face on your profile that is not actually your own - celebrity lookalikes are very popular. Therefore, extending the police database to include personal profiles will result in high levels of false allegations.

Here are some basic rules for using commas:

  • to separate items in a list: There are different types of societies: primitive, democratic, tribal and socialist.

    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. For example: The internet has redefined social interaction, information consumption, business practices and travel arrangements, and has also transformed the whole process of banking.

  • to join two related sentences, the second of which begins with and, or, but, yet or while: Many commentators think that the internet promotes democracy, but others are not so sure.

  • to mark the beginning and end of a sub-clause - a subclause is a phrase that provides additional information: An open society, as defined by Karl Popper, is based on the activity or many individuals.

  • to mark the beginning or the end of direct or quoted speech: 'Internet technology has facilitated instantaneous communication', says Noel Sarah Dietrich.

    If you put commas in the wrong places, or miss them out when they are needed, your tutor may find it hard to understand what you are trying to say. This will cost you marks. If you are unsure about a comma, read your work out loud and see if you can hear where the commas should be.