 <p>While they weren't allowed in c89, they are allowed now. The main difference between the two is the number of lines your comment needs. But generally:</p> <pre><code>int x = 0 // This comment can be used for quick descriptions. /* * This kind of comment should be used for multiple lines * that take more than 3 lines. */ </code></pre> <p>In the end, it all depends what you prefer, but this is general convention that I've seen on how to use each comment.</p>