Compound vs. Complex Sentence [3 Differences]

Can you tell which of the two sentences is compound and which is complex?

Although he claimed to have won state level championship in chess, he struggled to make even basic moves.

He claimed to have won state level championship in chess, but he struggled to make even basic moves.

And now?

I asked my friend to lower the volume of music he was listening because it was loud.

I asked my friend to lower the volume of music, but he didn’t, so I left the room.

The first in each set is complex, and the second compound. In this post, we’ll look at the difference between the two types of sentences from the point of view of not just how they’re constructed, which BTW is a superficial way to contrast the two, but also what they do in writing.

These are the three areas where compound and complex sentences diverge.

1. They’re constructed differently

Let’s get the simple part out of the gate first.

Whereas a compound sentence contains two or more independent clauses, a complex sentence contains one independent clause and at least one dependent clause.

Here is a breakup of dependent and independent clauses in the four sentences we just saw (dependent clauses underlined).

Although he claimed to have won state level championship in chess, he struggled to make even basic moves. [1 Dependent clause + 1 Independent clause]

He claimed to have won state level championship in chess, but he struggled to make even basic moves. [2 Independent clauses]

I asked my friend to lower the volume of music he was listening because it was loud. [2 Dependent clauses + 1 Independent clause]

I asked my friend to lower the volume of music, but he didn’t, so I left the room. [3 Independent clauses]

Learn identifying clauses in a sentence:

The first and third fulfil the condition of complex sentence, and the second and third that of compound sentence.

But looking at only the number and type of clause in a sentence is a superficial way of understanding the difference between the two. If we lift the bonnet, we’ll find few functional differences between the two.

2. Complex sentences express one key idea; compound sentences, multiple

The main (or key) idea of a sentence is expressed by its independent clause(s), with the dependent clauses expressing a subordinate (or less important) idea. In this sentence, for example, his struggle to make even basic moves, captured by the independent clause, is the key idea of the sentence.

Although he claimed to have won state-level championship in chess, he struggled to make even basic moves.

Therefore, the next sentence in the sequence would build on his struggle, and not on his winning state-level championship. For example, the next sentence could be For example, he moved his queen out too early in the game.

A compound sentence with two independent clauses will express two key ideas. (This can also be understood in terms of coordinating conjunctions functioning as joiners of independent clauses, giving equal weight to both.) This sentence, for example, carries two key ideas, and you can write the next sentence in any of the two directions.

He claimed to have won state level championship in chess, but he struggled to make even basic moves.

What’s the implication of this?

One key idea per sentence makes for smoother reading. Pick any piece of writing from a reputed source such as newspaper or non-fiction book and count the type of sentences it contains. More often than not, you’ll find complex sentences far outnumbering compound and compound-complex sentences, sentences that contain two or more independent clauses.

For all the kick we get from the simplicity of compound sentences (just a comma and a coordinating conjunction) or their finite nature (only seven coordinating conjunctions) or the acronym FANBOYS, the fact is they’re not popular in professional writing.

There is another reason why they’re far less popular than complex sentences, which takes us to the next section.

3. Complex sentences can convey more information flexibly

Slamming the door on dependent clauses robs compound sentences of the opportunity to squeeze in extra information through a relative clause or to express a complex relationship through an adverb clause. FANBOYS, after all, cover a limited range of relationships compared to adverb clauses, and they’re a poor substitute for the other two dependent clauses.

Moreover, multiple dependent clauses, unlike independent clauses, easily fit into an independent clause like pieces of Lego.

An analogy to contrast the two types of sentences

An analogy to bring out the difference between compound and complex sentences would be merger and acquisition companies do to gain market share, technology, brand, product, etc.

Merger happens when two companies of similar size combine to form a new entity. For example, Exxon and Mobil merged to form Exxon Mobil Corp. In mergers, a new entity is formed, with management from both the merging companies finding representation in the board, the highest governing body, of the new entity. Mergers are rare.

Acquisition, on the other hand, happens when a large company co-opts (or buys) a smaller company. For example, Google acquired YouTube. In acquisitions, the acquiring company maintains its original self, with the acquired company becoming a small part of it at best. Acquisitions are common.

A compound sentence is like a merger. Two independent clauses of equal importance merge to form a new entity, where both find voice (two key ideas). And, like mergers, they’re rare in professional writing. A complex sentence is like an acquisition. A stronger clause (independent clause) co-opts a weaker clause (dependent clause) while maintaining its entity and voice (one idea). And, like acquisitions, they’re common in professional writing.

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