When Vermont and Kentucky joined the Union, two stars and two stripes were added to the flag, in keeping with the tradition set out by the original flag. However, as more states were going to be added thanks to the large territorial acquisitions made during the Napoleonic era, in 1818 congress passed a new Flag Act that mandated 13 stripes and a star for each state. For the record it was the flag act of 1794 that changed the flag to the 15/15 configuration.