Why should a pendulum be swung less than ten degrees?

Answer 1

The time it takes for a pendulum to make one swing is almost exactly the same regardless if it swings thru any small angle. Once the angle starts getting large, like more then 10 deg, the difference in swing time becomes noticable. If you use a pendulum as a clock,so each second is one swing, then if you start the pendulum swinging at about 10 deg it will continue to be one second per swing even as it runs down to a smaller swing angle.