Observational Parameters

Basically, the relative position of the stars could be evaluated by measuring the following parameters:

Position Angle (PA or theta) : in degrees, it is the angle measured from the North and toward the East, from 0° to 360°, until arriving to B, taking as vertex from the same one to the component A.

Angular Separation (AS or rho) : in arc seconds, it is the apparent distance that separates to the components A and B projected both on the celestial sphere.

If both stars have same apparent magnitude,it is considered component A the star that is more toward the North; in systems of more than two stars,a third component would be denominated C and so forth following the order of the Greek alphabet,and all position should always refer to the component A.

To learn more about binary stars, it is recommend to see some of the bibliography references. Particulary, to learn about orbital elements of double stars, let you see the "Course of Binary Stars" of the software EFEBIN, that you can download freely from this web site Binary Stars.