Grandsire Doubles is a fascinating and useful method. It has good music, it's not horribly difficult to ring and its calling can range from comfortably easy to nicely complicated. For many of us in the FSG area, it was the first method we learnt to ring, and therefore there is something very special about ringing it.
With regard to calling it, yes, there are the 10 extents: the six 2-parts and the four 3-parts (3rd & 5th observation). There are a number of 240s, which help to spice things up; and in a future article I shall list these. Also, there is the ability to change to 2nd & 4th observation (my article on this was published in the April 2002 Newsletter, No 95).
Quarter peals generally run to 1260 changes, which requires ten extents giving 1200 changes (or some 240s can be substituted, of course), and the remainder is made up of a touch of 60 changes. All this is nicely straightforward.
The odd 60 always seem to me to be rather untidy after plugging away at the extents: just a tacked-on, make-up piece, which of course it is. I usually like to hide the 60 somewhere in the middle of the quarter, so the performance can end more grandly with a 120 or 240. (My preference is to end with
the Old Friend: P B P B P S, repeated, 5th obs, as there's something particularly comforting about this one. You know it's so stable, it just can't go wrong!)
While trawling the net recently (as you do), I came across some touches of 180 changes. Huh, you might think: what's the point of those? Well, they are very useful, as they remove the need for the silly 60. Instead of calling an extent and then 60, you can just call a 180. Geddit? Much more elegant!
Here are the callings. There are 10 of them. All the repeated changes are repeated only once; and they occur once at handstroke and once at backstroke. You might notice the first three of the 2-part 180s look very much like the 2-part 120s, which is useful for remembering the callings, ie 2 plain leads in between calls instead of 1.