Pendulums have been used for thousands of years with the earliest known use as far back as ancient Mesopotamia. Due to the simplicity of such a tool, it would not be surprising if pendulum use goes back much farther than recorded history.
Documented uses include dowsing and construction. Pendulums were used to detect water and minerals. Pendulums were used as plumbobs to ensure straight and true vertical surfaces. They were also used in modern architecture to detect or show evidence of the sway of high-rise buildings.
Pendulums are an item hung from a string that is heavier than the string it hangs from so as to pull the string taught as it hangs. We now see pendulums of carved semi-precious stones dangling from a petite chain, sometimes decorated with charms or other, smaller semi-precious stones. It is easy to conceive that early pendulums were wood or rock dangling from catgut or some type of thread. The pendulum would react to water or minerals by swaying slightly back and forth or swing in a circular motion while hanging from the dowser's fingers. In the case of construction, it would be hung on the wall or from a window. Hanging on the wall, the pendulum would show how true the wall was to vertical; in a window, the pendulum would sway as the wind and natural forces swayed the building.
It is feasible that dowsing/ divination and construction uses of the pendulum developed simultaneously. Tools often had multiple functions and there was no separation of spiritual from the practical in daily life.
For simple divination, allow the pendulum to hang from its cord from your non-dominant hand. I allow mine to dangle between my non-dominant fingers. Allow the pendulum to hang and come to rest. Ask your pendulum for a yes- “give me a yes, please”. Now ask your pendulum to stop- “stop please.” Ask your pendulum for a no- “give me a no please”. Finally, ask it to stop again. Now you are ready to ask your pendulum yes or no questions and receive its answers.
Though these explanations are simplified for the purpose of a brief introductory synopsis, there is plenty of information available for further exploration both online and in written text.
Enjoy your pendulum!