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The semi-colon is often used to join together two independent clauses — in other words, it joins two clauses that could be sentences.
For example:
I am proceeding to the conference without her; She is running late.
However, when we use a semi-colon, we are usually suggesting that there is a relationship between the sentences, but we are not making that relationship clear. Usually, you can tell from the context what the relationship is.
When we use a semi-colon, it is often because we want to make the readers think about the relationship for themselves.
There are other uses of the semi-colon which will be discussed again next time.