The above image is a teaching aid found in the book, The Brand Gap by Marty Neumeier.
"Swap part of your logo—the name or visual element—with that of a competing brand, or even from another brand category. If the resulting icon is better, or no worse that it was, your existing icon has room for improvement. By the same token, no other company should be able to improve it’s icon by using parts of yours...A good brand icon is like a tailored suit—it should only look good on you. Another test is too remove a piece of your logo such as the name. Would people still be able to tell that it is your brand icon? If the remaining piece is looks like it could have come from any other company or brand, then it is not serving it’s purpose."
From the highly recommended book, The Brand Gap.