A plaid shirt is a shirt with a pattern of crisscrossing horizontal and vertical bands of color, known as plaid or tartan. Plaid is not a fabric, but a pattern that can appear in any number of fabrics and colors. Flannel, on the other hand, is a fabric that is often used to make plaid shirts, but not all flannel shirts are plaid. Flannel is a warm, comfortable fabric traditionally made of wool, but also commonly made of cotton. Plaid shirts are popular due to their versatility and can be worn with jeans for a classic rugged look, chinos for a preppy look, or even yoga pants for a relaxed look. Plaid flannel shirts became popular in the early 1990s as part of the grunge style of bands like Nirvana and Pearl Jam. In summary, a plaid shirt is a shirt with a plaid or tartan pattern, while flannel is a fabric that is often used to make plaid shirts but can also be found in a wide array of patterns or solid colors.