These two are as easily mixed up as stationary and stationery.

Compliment and Complement

I regularly find the Kano model a useful way to think about design.

Kano highlights 3 different types of benefits your product can provide:

Threshold
  These support the minimum requirements of the product. Like the brakes on a car, you’re not going to buy one without them, and yet no matter how good you make them they are unlikely to thrill people about the car. But if you do a bad job, people will sure notice.


Performance
  these benefits are appreciated the more you deliver on them. For a car this may be acceleration or fuel efficiency. The better you make them the happier people will be, and the worse they are the least satisfied they’ll be.


Delighters
  these are the fun benefits that get people to love a product, but if you only had delighters you wouldn’t have a product. For instance, if you car is slow, not fuel efficient and the brakes don’t work, it doesn’t matter if it has the best stereo on the planet, for almost everyone it just won’t help. But if your car has serviceable brakes, reasonable acceleration and fuel efficiency, and the best stereo on the planet then it could really delight.

Use both sides of a towel