When a pendulum clock is in beat, it produces a rhythmic, even, steady:
When it is out of beat, it produces an uneven, “galloping”:
If it is badly out of beat, the uneven rhythm is quite obvious. But as it gets closer to in-beat, it gets closer to even, and you may have to listen carefully, for a while, to decide whether it’s really even or just close. If your clock is far out of beat, the clock will probably stop after a few minutes.
Why does this happen?
Setting the beat of a grandfather clock is done the same way. Listen to the tick tock and change the clock leveling to to the left or right until an even tick tock is heard. You also may remove the pendulum and move the crutch with your finger to the left or right until the proper tick tock is reached.