single thread. and a ladder stitch- check out the tutorials, or u-tube videos there are lots of them showing exactly how to do nice small-invisible stitches for your bindings. i often use silk thread because i can use a 100 wt thread (which is very fine/thin but strong) or a 60 wt thread---the higher the wt number the thinner/finer the thread--as in 12 wt is like string--100wt is very fine- often we piece with 40-60wt threads.