The Prusik knot. Where would kite-fliers be without this great shiftable, lockable knot... A bit of a pain to tie at first, but it makes adjusting a bridle so easy! Pulling the bridle straight unlocks the knot, letting you shift it along the bridle line. Pulling on the lines as pairs as in the last photo causes the knot to fold, locking it in place.
I use the Prusik on horizontal bridle loops to shift the knot left or right across the kite, for trimming purposes. More commonly, it's also used to slide up and down the bridle line of, say, a Diamond kite to adjust the Towing Point fore and aft.
On kites with 4 legged bridles, arranged as an upper and lower loop plus a connecting line, this means 3 shiftable knots. For example, the Dowel Rokkaku and Dopero. At least you only have to tie these knots once.
Although it seemed a little excessive at the time, I took this approach on even my smallest Dopero. A tiny kite with a 30cm (12") wingspan. But it sure paid off, since some fiddling was required to get a decent flight out of it!
More recently, I have also used this knot on the vertical bridle loops of a large Genki. Same deal, tiny adjustments can trim the kite left or right.Now, if you really want to go overboard with knot-tying...
There are other ways, some possibly a little simpler, to obtain a shiftable knot. However, the Prusik has worked so well for me over the years that it's the one I would recommend.
This mountaineering knot was invented in 1931 by Dr. Karl Prusik. With one 's'. At least this web-master has finally got it right. Many others still haven't! Prusik later authored a mountaineering manual, which was the first publication to feature this well-known sliding knot.
Tying a Prusik knot actually becomes fun after a while, funnily
enough! Be sure to put a Simple knot into the short end of the line
though, since constant use tends to pull line through the knot. The
short end gets noticeably shorter than you remember tying it originally.
Perhaps this happens during locking and unlocking...