Over the last ten years it has become more and more popular for rope teams on glaciers to tie knots between one another. The idea is that should someone fall into a crevasse, the rope will cut into the lip and one of the knots will get stuck, thus arresting the fall.
We teach a lot of crevasse rescue at the American Alpine Institute and enjoy testing different theories while we're in the field. Most of our guides have done some level of testing on this particular glacier travel theory and amazingly enough, it works...sometimes.
What we have found is that there are two types of knots. There are knots that are flat on one side and knots that go around the rope. Knots that are flat on one side, like an overhand or a figure-eight on a bite, tend to slide over the lip more easily than knots that go around the rope, like a butterfly.
In our testing, what we've found is that early in the season, when there is more snow and the snow is softer, figure-eights and overhands will often bite the lip and hold. But as the season progresses and the lips become icier, the knots just slide right over. Butterfly knots are more likely to bite into the lip both early in the season as well as later.
The following video shows a demonstration of how to tie a butterfly knot:
There are some disadvantages to knots on the rope between climbers. When there are a lot of sastrugi formations or penitentes on the snow's surface, the knots can get caught and will hinder movement. It can be difficult to haul a person out of a crevasse who is being held by a knot as you will have to pass the knots. It can also be difficult for a climber to prusik out and deal with the knot welded in the lip.
I generally don't put knots in the rope on teams of four or more. There is so much weight in the system that it really isn't required. Three person teams are a little more difficult. If they are experienced, I usually don't put knots in the rope, but if they are novices, I'll usually put a couple knots in the rope. On two person teams, I always put butterflys in the center of the rope.
It's better not to put too many knots in the rope. If there really is a crevasse fall, they might arrest a victim, but that doesn't mean that it will be easy for the person to get out. Instead, most guides put one to three knots in the rope between themselves and the other climbers. More than that generally just creates more problems.
Knots in the rope are a nice additional safety measure, but they will not take the place of good technique and a solid set of skills.
--Jason D. Martin