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:

To summarise there are various tectonic settings at which different volcanism occurs this essay has identified and described some kinds of volcanic activity that has taken place in each of them it details the input processes and outputs.

This sentence should be divided up as follows:

To summarise, there are various tectonic settings at which different volcanism occurs. This essay has identified and described some kinds of volcanic activity that has taken place in each of them. It details the input processes and outputs.

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 study of Earth Science has redefined our knowledge of volcanoes, the ocean floor and tectonic plates, and has also fed directly into our understanding of climate change.

to join two related sentences, the second of which begins with and, or, but, yet or while: Many commentators think that carbon warms the atmosphere, 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: Hot spots, which are located throughout this planet, are areas where more than a normal amount of magma is produced.


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.