A semicolon is not the same as a comma. They are not interchangeable. Both the comma and the semicolon indicate pauses, but the pause indicated by a semicolon is generally longer and is closer in intent to a full stop than a comma. This is why you should think of a semicolon as comprised of both of these punctuation marks (it certainly looks like both of them together). 

If you can finish a clause so that it makes sense on its own - if it could stand alone as a sentence - then you can end it with a full stop/period. However, if you would like to connect another clause so that it explicitly continues your line of thought, and is in fact needed to expand upon the first clause, then you can use a semicolon to separate these two clauses. You need to be sure, however, that your semicolon is in the correct place. Let me give you an example:

A good idea would be to take note of your surroundings, for example; the layout of the building.

This example is grammatically incorrect. The semicolon here is misused, and it has been mistakenly thought of in terms of a colon. So, there is more than one issue here.

First, look at where the semicolon is. Everything up until that point could logically form a complete sentence which would make sense on its own; like this: A good idea would be to take note of your surroundings, for example.

The problem is that the rest of the sentence is now a clause on its own: ...the layout of the building. It doesn't make any sense like this, so what should we do with it?

We could form two separate sentences:

A good idea would be to take note of your surroundings, for example. You should note the layout of the building.

Using a semicolon allows you to be more concise, and it helps you to avoid sentence fragments and constant stops and starts for your reader.

So, the ideal solution for this sentence would be as follows:

A good idea would be to take note of your surroundings; for example, the layout of the building.

In this solution, 'surroundings' is treated as the word that could logically have completed the first clause as a single sentence. 'For example' could logically open a new sentence, but the relationship between the content of the two clauses allows us to combine the clauses and get our pauses and our line of thought in order.

Now, imagine that 'the layout of the building' was only the first item in a list. How do you think that sentence structure would go? Lists are convenient ways of condensing a lot of information; and in writing, they are often best delivered by using semicolons to separate the individual points of information or items on the list. Leading your reader to a list can be achieved in a variety of ways, but if you want to include a list in your sentence structure you must first know how to: 1) introduce it; 2) separate it with semicolons; and 3) finish it correctly.

Did you notice our list? Did you see where the semicolons were placed? The numbers used above are optional, and you might see this approach used in academic work or in business contracts. It helps to guide the reader through complex lines of thought and argumentation. This could be achieved in bullet point form, but first we want to show you a sentence containing a list. Remember, that a bullet point list is just a sentence which has been broken down into a variety of options for completing that sentence. The same is true of a list in a sentence; it's just a different way of doing it.

Let's look at our example with some new options included:

A good idea would be to take note of your surroundings. For example, you should note the layout of the building; the position of the fire exits; and the number of people working there.

There are a number of things to note here.

1. We have returned to the two-sentence structure. This is because using the semicolon before 'for example' and then again (repeatedly) in our list would be too much. This would confuse the reader as to the significance of the semicolon.
2. We have added 'you should note' because without it our new second sentence would not be grammatically correct. Remember that every self-contained sentence is exactly that. Although it might be out of context, it should still make sense without the sentences surrounding it.
3. We only need a comma after 'example'. This is because our list is about to start, and the style we have chosen it instructional: 'you should note...'. In this case, we could start a sentence with 'You should note', but because we are only giving examples (and not instructions covering the whole range of options), we are making this clear.
4. We use 'and' following the last semicolon. This should only be used before the final item on the list, and you should always end your list after you've used it.

There is an exception to this final point, however. If there is a grouping/pairing of items ending your list, the semicolon is placed before the group/pair, and the group/pair is separated by either 'or' or a comma. The list would end one of two ways, as follows:

For example, you should note the layout of the building and the position of the fire exits; and the number of people working there and where they are positioned.

For example, you should note the layout of the building and the position of the fire exits; and the number of people working there or where they are positioned.

Obviously, the first of these two examples makes more sense, but you can see how the placement of the semicolon separates the pairs of items.

Now we have come full circle, because we can return to our opening sentence and change it into a list; this time using the colon where the writer mistakenly placed a semicolon:

A good idea would be to take note of your surroundings, for example: the layout of the building; the position of the fire exits; and the number of people working there.

In this example, the colon introduces the list that follows; and the list is indicated by the semicolons used in between each of the three items. The way to think of these lists is as a collection of items which could logically complete the opening clause if any of the other items were not there at all. This brings us back to our solution to the shorter sentence: A good idea would be to take note of your surroundings; for example, the layout of the building. Any one of the three options above could follow the comma and the sentence would make sense. It's all about making sense and not leaving your sentences open to interpretation.

You could also show this same information in a bullet point list, using semicolons as follows:

A good idea would be to take note of your surroundings. For example:

• the layout of the building;
• the position of the fire exits;
• and the number of people working there.

Note, however, that not all English styles create bullet point lists in this same way. Some styles don't use punctuation marks at the end of the points; and some would not even complete the sentence options with a full stop. This issue is covered in our page on the full stop/period.

Note: If you are ever finishing a list in a sentence with 'etc.' (etcetera), make sure you put a comma before it - it is treated as the next/last item in that list, because it is a substitute for the remaining items you have chosen not to mention. (Also, treat the . in etc. as a full stop)