Liz Carroll sez that "Single jigs are also known as 'slides'".
She described the difference in terms of the steps a dancer does in
each. if I remember correctly: in a single jig, the steps are done on
one side. In a double jig, this figure is repeated on the other side.
This can be heard in the playing, although it is very subtle.
Single jigs are played something like this:
JIG-itty JIG-itty JIG-itty JIG-itty . . .
Whereas Double Jigs are played:
JIG-itty Jig-itty JIG-itty Jig-itty . . .
(the emphasis follows the capitalization)
This may be why people say that double jigs are in 12/8 time, and
single jigs in 6/8, but I have seen many books that treat it just the
opposite (having the double jigs in 6/8 and slides in 12/8).
Am I making any sense?