How do they calculate star distance?

[Pyrotex]

Astronomers using a network of radio telescopes stretching from Hawaii to the Caribbean have refined measurement of the distance of the Milky Way spiral arm nearest Earth, confirming that it is only about half as distant as previously estimated. ...A team headed by Mark Reid of the Harvard-Smithsonian Center for Astrophysics used the Very Long Baseline Array (VLBA), which delivers the highest resolution of any telescope, to determine the distance to the Persius arm of the Milky Way, previously believed to be 7,200-14,000 LY from the Sun's orbit. By measureing the motion of a newly formed star there designated W3OH with an accuracy of 10 micro-arcseconds -- 100 times better than possible with previous methods -- the team determined with trigonometric parallax that the the arm is about 6,400 LY from Earth, +/- 2%.

Wow. 6,400 Light Years away with an accuracy of +/- 2%.