In reply to Pursued by a bear:
Mainly used for sport climbing and indoors where the routes are very steep. They comprise of a prism on a frame that allows you to watch the leader without raising your head. Essentially you just look straight ahead. Great for preventing "belay neck". They also tend to encourage the belayer to focus more intently on the leader, which was a by-product that I had not anticipated but is welcomed. Once upon a time you looked a prat but these days many people wear them. i wouldn't be without mine although I don't wear them for trad. The original ones were on a frame, just like spectacles, but made wearing them with spectacles a little awkward. Now you can buy them as clip-ons, just like clip-on shades but more bulky as they are hinged so that they can be lifted out of the way when not looking up. I now wear these in preference.
Al
Post edited at 17:00