Why do the stripes on a flag go horizontal not vertical?

All flags represent an organization or institution of some kind. The shapes and symbols, and their colors, on the flag represent that organization's core principles. The US flag represents the US government. The core principle being expressed in the stripes is "E pluribus unum", that is, "one from the many". The stripes are the "many" part, the constellation of stars is the "one" part, forming a single united, or federated, constellation. Originally there were 13 states (countries) confederated into a United States, hence 13 stripes, and as various versions of the flag evolved from prior ones, often a stripe as well as a star were added when a new state joined the Union.

Now to return to the question of the stripes' orientation. The flag represents the US government, or some agency or aspect of it. By convention, horizontal stripes denote the US government in a military capacity. Vertical stripes, and there have been several flags that actually had vertical stripes, were adopted to denote the government in a civil capacity. So the version of the flag that currently prevails is the _military_ version.

Citation: http://www.civil-liberties.com/pages/mystery_of_the_flag.htm