Thanks to the invention of a clevis pin, you can secure two pieces together and yet they can still be movable. Whether it is from side to side or from up to down and vice versa, it will depend on the position of the connecting parts.

Imagine a train. It usually has more than two cars. Between cars they are connected but when the train has to make a turn it should be able to move from side to side. It also makes detaching and attaching another car easier. It is often used in rigging for the sails to be easily moved and yet securely attached.