Plaid (pronounced “plad”) is a common textile pattern formed by criss-crossing horizontal and vertical strips of colors. In Europe, the pattern is more commonly referred to as Tartan.
The plaid pattern has its origins in Scotland, where it was the primary form of fashion as early as the 16th century. In the early 19th century, the pattern came to fashion among the nobility when King George IV visited Scotland and brought the pattern back to Britain.
More recently, the Punk subculture in Europe adopted plaid in the 1970s as a form of rebellion against authority and the establishment. In the United States, the simple plaid pattern has come to be associated with farmers and cowboys, and plaid is increasingly common within the hipster fashion scene as well.
Because of its many connotations, wearing plaid can be tricky. First of all, avoid wearing green plaid, especially plaid pants, unless you want to look like you're wearing a Scottish kilt.
Second of all, because of the possible negative connotations of wearing plaid, make sure the fit of your clothes is good. If you wear oversized plaid shirts (especially if they're Flannel), you can come across as a lumberjack, country hick, or just plain tacky.
Finally, because plaid shirts can be very colorful and dynamic, they make good pieces to have when layering with other clothing items.
Of course, plaid is a very versatile pattern and can be used in many places. Here are a few more examples of plaid in action: