Comment by ks2048

I think an intuitive way is: Your starting point is on a circle at angle theta_1 and you want to draw an arc of length delta_theta (positive or negative). You could define the circle via center (x,y) or radius.

Edit: it looks like the SVG docs include the math for converting to and from this representation,

https://www.w3.org/TR/SVG/implnote.html#ArcConversionEndpoin...