mainly pendulum has 3 principles.
1) The time period of pendulum is independent on mass of bob of pendulum
It may be wood pendulum or metal bob pendulum the time period remains same.
2) the angle of suspension less than 5 deg. Then only the motion of pendulum is said to be Simple harmonic motion.
3) the time period of pendulum depends on Length of pendulum.
The time period is directly proportional to square of length of pendulum.
I hope it is clear to you.