Better sentence-writing in 30 minutes a day - Dianna Campbell 1995

A note on punctuation
Complex sentences
Sentence combining: basic strategies and common problems

Complex sentences are another easy technique of sentence combination, and they provide even more variety in your writing because most of them can be presented in two different sequences. In a complex sentence, the conjunction can be placed between the clauses, just as in a compound sentence. For example:

(a) The mandrill of Western Africa is often called the most colorful mammal in the world because it has a brilliant crimson nose and bright blue cheeks.

Or the conjunction (the word because) can be placed before the first clause in a complex sentence. For example:

(b) Because it has a brilliant crimson nose and bright blue cheeks, the mandrill of western Africa is often called the most colorful mammal in the world.

In both the (a) and (b) sentences, the clause that comes right after the conjunction is called the dependent clause. It’s called dependent because it depends upon more information. It can’t stand alone. Because it has a brilliant crimson nose and bright blue cheeks doesn’t make sense by itself. It needs something else, namely an independent clause. Once it is attached to an independent clause, such as the mandrill of western Africa is often called the most colorful mammal in the world, the two clauses work together to make a perfectly good sentence—a complex sentence.

A note on punctuation

If you’re especially observant, you may already have noticed that a comma is used in the preceding examples only when the conjunction appears at the start of the first clause; then a comma is placed between the dependent clause and the independent clause. A comma is not generally used in a complex sentence when the conjunction appears before the second clause; in other words, you don’t use a comma when the conjunction is in the middle of a complex sentence. Here is another way to say this:

dependent clause first → comma between clauses

independent clause first → no comma

There are exceptions to this rule, and later you might want to learn about them and about some other fine points, but in this book, we are concerned with the basics, and this punctuation rule is correct for the vast majority of complex sentences that you will write.