Fashion is defined as the prevailing clothing style during a particular time. That definition suggests that fashion changes over time. What makes a dress or an accessory trendy is not rocket science. But how do trends emerge?

Trends have no single source. Their origins can be traced to movies, street culture and social movements. Industry professionals say artistic creativity flourishes when people take risks, are open-minded and determined.

How many people need to be wearing a yellow skirt or haute scrunchies before they can be called fashionable? In many cases what matters is who is wearing them. Styles can become trends when mass media publications, start writing about them.

The advent of the movies in the early part of the last century made people more aware of style and trends, and the increasingly affluent middle class could aspire to look like film stars. As a result entertainers became fashion influencers.

But how can you tell if it’s a trend or a fad? If it keeps evolving and crosses over from one group of consumers to another over a period of time, then it’s a trend. If it doesn’t progress after it first became popular, then it’s a fad. (Think butterfly hairclips and wallet chains.)

The following list is a selection of Americans who have had tremendous influence on what people wear, despite not being fashion designers themselves. You will not see names such as David Bowie and The Beatles on the list because they were not American, even though they influenced fashion trends all over the world, including the United States.

Click here to see America’s most popular fashion influencers.