The modern United States flag has 13 stripes, representing the original thirteen colonies.
The stripes alternate red and white and run horizontally across the flag. Their number has remained 13 even as the country expanded from the original colonies to 50 states. The blue canton contains the stars representing the states.
The Flag Act of 1777 specified thirteen alternating red and white stripes and thirteen white stars on a blue field. Congress later changed the flag by adding stars and, temporarily, additional stripes as new states joined the Union.
In 1818, Congress returned the design to 13 stripes and provided that future additions would be represented by stars only. This explains why the current flag combines an unchanging group of stripes with a star field that grew over time.