The Compound-Complex Sentence

July 22, 2025

Posted by STLCC in Writing Resources 

The compound-complex sentence is composed of at least three clauses (each with a subject and verb): at least two independent clauses and one dependent clause. This more advanced structure allows the writer to show relationships between multiple ideas.

You might think of this as a sort of verbal equation. In the patterns below, each clause contains a subject and a verb. The examples demonstrate how the clauses work together to form a compound-complex sentence.

Pattern:
A dependent clause followed by two independent clauses.
(Subordinating conjunction + subject + verb), subject + verb, coordinating conjunction + subject + verb.

When they observed fossils and rock strata, ancient humans puzzled about them, but no one understood their significance.*

Changing the Order of Clauses

The equation can be organized in other ways as well. The dependent clause may modify the second independent clause, so it should appear as close to that clause as possible. In this case, the dependent clause comes at the end of the sentence.

Pattern:
Two independent clauses followed by a dependent clause.
Subject + verb, coordinating conjunction + subject + verb (subordinating conjunction + subject + verb).

Aristotle, Avicenna, and Leonardo da Vinci speculated about geology; however, fully formed theories weren't developed until the eighteenth century, when Abraham Werner and James Hutton devised theories at about the same time.

Or, the dependent clause may modify the first independent clause and be placed nearest to it, in the middle of the sentence.

Pattern:
An independent clause followed by a dependent clause and a second independent clause.
Subject + verb (subordinating conjunction + subject + verb), coordinating conjunction + subject + verb.

Werner's theory hypothesizes that all Earth's rocks were deposited in a global flood, so it is often referred to as the Neptunist theory.

Using More Than Three Clauses

Added to this flexibility is the fact that a compound-complex sentence might contain more than three clauses. Additional dependent clauses or relative clauses can provide more detail while still connecting related ideas.

Example:

Although some of Werner's terminology is occasionally used by geologists today, the Neptunist theory has been supplanted by James Hutton's Plutonist theory, which suggests that all rocks are being constantly reformed by heat in the Earth's core; in fact, Hutton is often considered the father of modern geology, since most of the ideas that he formulated have proven to be correct.

Let's practice on something a little less convoluted.

Practice

Here are three related sentences:

1. During the Cambrian Era, none of the continents existed as we know them today.
   (complex sentence)
2. Sea level was much higher.
   (simple sentence)
3. Very little water was tied up in ice.
   (simple sentence)

While it is acceptable to leave these three sentences separate, the reader has no hint of the relationships between them. The two main points seem to be about the continents and the sea level, so let's connect them with a coordinating conjunction:

1. During the Cambrian Era, none of the continents existed as we know them today, and sea level was much higher.
2. Very little water was tied up in ice.

The final sentence helps explain why the sea level was much higher, so it makes sense to attach that idea to the compound sentence with a subordinating conjunction:

1. During the Cambrian Era, none of the continents existed as we know them today, and sea level was much higher because very little water was tied up in ice.

*All examples based on information from "Geological Time Scale." Wikipedia. Wikimedia Foundation. 26 March 2015. Web. 31 March 2015.

Learn more about complex sentences.