Introduction to Comma Rules

Look at any grammar resource. Two points become very obvious.

  1. There are many “rules.”
  2. The resources do not necessarily agree.

Off course, add the different nuances in different English-speaking countries and it becomes even more interesting.

At first glance, comma rules may seem annoying and confusing. As one gets used to them, though, they actually start to make a lot of sense. The big thing to remember when using commas is that all important basic sentence unit, namely, the complete sentence (subject, verb, and object).

When using a comma, that basic unit should not be disrupted in any way, either by separating essential elements or creating sentences that are too long. Basically, it’s easier to think of commas as being used to indicate extra information. This can take place within a sentence, but never in such a way as not to disrupt the basic sentence unit. 

A handy tip for spotting when a comma is being used incorrectly is to cover up the different parts of the sentence to see if one of the parts makes sense without the part that is being covered.

Let’s look at an example:

Monozygotic twins, are genetically identical.

Let’s cover the second part of the sentence:

Monozygotic twins …

Clearly that part is not a complete thought. Let’s try the next bit:

… are genetically identical.

This does not form a complete idea either, so the comma has to be removed:

Monozygotic twins are genetically identical.

Here’s another example:

Even when identical twins are raised apart, they grow up very like each other.

So, let’s start by covering up the last part of the sentence:

Even when identical twins are raised apart …

Clearly that part is not a complete thought. Let’s try the next bit:

… they grow up very like each other.

Although it’s not clear who “they” are, the latter part of the sentence is complete. The first part, then, can be separated by a comma.

Here’s a last example:

Monozygotic twins, frequently referred to as identical twins, occur when a single egg is fertilized to form one zygote (monozygotic) which then divides into two separate embryos.

If the part between the commas is covered up, the outer part makes sense:

Monozygotic twins, …, occur when a single egg is fertilized to form one zygote (monozygotic) which then divides into two separate embryos.

The commas are free to stay in this case. ?

Be ready for lots more detail in the next few blog posts. In fact, the next blog will contain some comma rules.

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