Ken Kocienda recounts the process of designing the iPhone keyboard:
I started to think about improvements, and to help me keep my keyboard goal literally in sight as I sat in my office, I measured and cut out a small piece of paper, about 2 inches wide by 1.3 inches tall, a little smaller than half the size of a credit card turned on end. I pinned up this little slip of paper on the bulletin board next to my desk. I looked at it often. This was all the screen real estate I had available for my keyboard.