Okay, I don't usually get technical with punctuation, but having read a brilliant example on Twitter this morning, and having had this issue crop up with an assignment I was marking over the weekend, I thought it was worth mentioning.
When I was at school (which for me was many Prime Ministers ago), I was always told that we had to use a comma to separate words in a list. For example, if I'd been on a school visit to a local farm, the teacher may have asked me to write about it. As a seven year old, I'd have written something like this:
Last week I went to the farm and I saw cows, pigs, sheep, goats, horses and geese.
My teacher would have praised me for using a comma to separate each item in my list of animals. I would also have been praised for NOT using a comma after the word 'horses' and before the word 'and'. I was always told at school that we did not need a comma before an 'and'.
BUT ... there are times when you do, and it is known as the serial comma, although many people know it as the Oxford comma, because it is a stylistic point used by the Oxford University Press.
Which brings me to the great example mentioned on Twitter this morning. Here is a sentence that has a list and does not have a comma before the word 'and':
"I dedicate this book to my parents, Martin Amis and JK Rowling."
Reading this sentence as it is, it suggests that this author is the love child of Martin Amis and JK Rowling! Now, I'm sure this isn't the case ... Martin Amis and JK Rowling may never have even met ... but this sentence, as it stands, tells us that this book is dedicated to two people.
To clarify the sentence, we need to insert the Oxford comma, before the word 'and', like so:
"I dedicate this book to my parents, Martin Amis, and JK Rowling."
Suddenly, all becomes clearer! Instead of dedicating the book to two people (Martin Amis and JK Rowling), the book is now dedicated to four people (Martin Amis, JK Rowling and both of the author's parents). The Oxford comma may break the rule that I was always taught at school, but it clarifies the sentence, which is what all good punctuation should do.
A comma may only be a small squiggle of ink, but sometimes, it's an important squiggle!