lace curtain stretchers have short sharp nails that are driven in from the back.
the frame, when put together, forms a rectangle. think flat. the four pieces of wood are studded with the nails. the two side pieces are very long and allow you to adjust the length of the frame to fit the length of the lace curtain. the nails are close together so you don't get little dips in the lace fabric when it dries.
while the fabric is still wet, you would push the top of your lace curtain evenly onto your stretcher, pulling on the width as you go. then you would attach the bottom end to the other short end of wood, which is still loose. when the lace is evenly distributed, top and bottom, the fun really begins.
you then pull with all your might until the curtain can't stretch any more. that's when you tighten the sides to adjust the length and lock it in. at this point you pull on the sides to stretch the long edges of the curtain onto the long sides of the stretcher frame so you can adjust the width of the curtain.
you work your way down, top to bottom, or bottom to top, both sides at the same time to stretch evenly. usually two women did this. you can see why.
since lace was made of all cotton , it stretched a lot, but with a stretcher, you could keep exactly the right size. you could buy them ready made or your husband made them for you. if you had sheers, you could still use them, but the weave had to be loose enough.
they were used primarily during the 1800's and thereabouts. around the time of victorian houses. they had the very tall windows and you couldn't afford to lose even one inch of length in your curtains (actually drapes if you want to get technical) or they didn't look right. also, remember how expensive all that cotton lace must have cost. you wouldn't want to ruin it.
my friend, who lived many years in a victorian home, actually did this. i could not believe my eyes. she and her husband made a whole hoo-haa every two years. when they had a furnace blow-back, i thought they would have to be committed. they came to their senses, sold the house (with curtains) and bought a ranch. they're much happier.
any questions?