Here's why:
* Compound sentences are made up of two or more independent clauses. An independent clause is a group of words that can stand alone as a complete sentence.
* Coordinating conjunctions (like "and", "but", "or", "nor", "for", "so", "yet") connect two independent clauses.
* When you don't use a coordinating conjunction to connect the clauses, you need to use a semicolon to show the reader where one clause ends and the next begins.
Example:
* Incorrect: The rain poured outside the window the wind howled.
* Correct: The rain poured outside the window; the wind howled.
Note: Sometimes you can use a comma instead of a semicolon, but only when the clauses are very short and closely related. However, using a semicolon is generally a safer bet.