Discworld novels often included a modern innovation and its introduction to the world's medieval setting, such as a public police force ("Guards! Guards!"), guns ("Men at Arms"), submarines ("Jingo"), cinema ("Moving Pictures"), investigative journalism ("The Truth"), the postage stamp ("Going Postal"), modern banking ("Making Money"), and the steam engine ("Raising Steam"). The "clacks", the tower-to-tower semaphore system that sprang up in later novels, is a mechanical optical telegraph (as created by the Chappe brothers and employed during the French revolution) before wired electric telegraph chains, with all the change and turmoil that such an advancement implies. The resulting social upheaval driven by these changes serves as the setting for the main story.