just to add to mike and ian's post.
single trim or mono filament: bristles all same length and supposedly less chance of spotting as bristles are not meant to trap dirt within themselves.
flocked bristle brushes were designed originally for cleaning viechles, the ends of bristles are softer so wont scratch body work on a car.
not really designed for cleaning windows but will just rinse well, but some say bristles will trap dirt, more chance of spotting.
Double trim brushes have longer bristles on outside and shorter in middle,
this design allows you to rinse on the glass.
Up and down over glass then finish with side to side motion down pane;
instead of lift and rinse, quicker and saving a bit of water also i'd imagine.
I've tried all and at moment i am happiest with vikan hi-lo mono-filament(red bristle) sill brush

but i am intrested in alex gardiners sl brush (think it's called) superlite double trim brush, not been impressed with double trims in past but heard good things about this one.
Alex is i think working on a superlite sill brush have heard

wonderer i this is single or double trim

thanks lee