The words are used loosely, so here is how we use them.
A poncho, in its oldest form, is a square or rectangle of cloth with a slit in the middle for the head. It comes from the Andes, and the word reached English through Spanish from Mapuche, the language of southern Chile. In the mountains it is still worn that way, in thick wool.
Ours are cut differently. There is no opening for the head: the front is open all the way down, closer to the ruana of Colombia than to the classic poncho. It settles on the shoulders and falls in two long panels, and you can wear it open, over a shoulder or belted.
A cape is shaped to the body. It sits on the shoulders, usually fastens at the neck and has openings for the arms. Our capes have collars, hoods or buttons and fall to the knee or the floor; the Royal Garden Cape falls to the ankle and buttons from neck to waist, under leaves embroidered by hand.
A shawl is a single piece of cloth, square, long or triangular, worn over the shoulders or the head. The finest came from Kashmir, woven by hand; Europe took to them after Napoleon’s campaign in Egypt in 1798, and Paisley in Scotland was weaving its own from 1808.
So: a shawl you hold, a cape you fasten, and a poncho, ours at least, simply falls. See the ponchos · See the capes