Frank Sinatra was known for giving serving staff crispy $100-bill tips. That was a long time ago when a hundred dollars really meant something. What are the rules for tipping today, and why are customers confused?

Many people wouldn't think twice about tipping a waiter in a nice restaurant, the hairdresser, a good bartender or the porter carrying heavy luggage through a busy hotel. These are situations with clear, long-standing norms in many countries.

But what about the barista at Starbucks? Or the person taking your order at a fast-food takeaway window? What about a self-service kiosk?

Most historians agree that tipping started in medieval Europe with aristocrats handing out gratuities to servants or those who worked their land.

By the 19th century, the idea was disappearing in Europe but had arrived in the US. Later, it was re-exported around the world.

Today, people tip for any number of reasons: To feel better about themselves, to impress others, to help make up for paltry service-staff pay or because they are asked to.

Tipping is primarily driven by motivations to help servers or reward good service, says Michael Lynn, a professor of services marketing at Cornell University in the United States, who studies tipping.

Others tip to fulfill a perceived obligation to tip, Lynn told DW. Still others are more self-interested. These people tip to gain or maintain future preferential service or social approval, said Lynn, who is currently writing a book on the subject slated to be called "The Psychology of Tipping: Insights for Service Workers, Managers and Customers."

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