(I think I first read this tip in the British climbing magazine “High” about 15 years ago, and I believe the article was written by Andy Kirkpatrick, so I‘ll give him credit here for the idea.)
There are various approaches to keeping your ropes tidy on a multi pitch climb. Methods like laying butterfly coils across your lap or placed into a doubled over sling can work pretty well, but unless you’re attentive to making tidy coils, you can make a mess pretty quickly.
Another approach is using a Metolius Rope hook, a clever bit of gear we cover in this tip.
Here’s an alternative that’s useful in both standard climbing and big walling.
Take a widemouth locking carabiner, such as the DMM Boa or Black Diamond Rocklock, and clip somewhere onto the anchor.
As the second climbs up and the belayer takes in slack, the belayer ties an overhand knot every 30 feet or so and clips the loop to the big carabiner. Keep these overhand loops loose, so they are easy to untie; don’t snug them down. For a 100 foot or so pitch, this will leave you with three or maybe four knots in the carabiner, each with a loop about 15 feet long hanging down from the belay.
When the second arrives at the anchor, the rope is in tidy loops and ready for the second to lead out. If you’re swapping leads, you’re all set to go, because the last loop to be clipped should be the first loop for the new leader to head out.
If you're leading in blocks with the same person leading several pitches in a row, you simply unclip the loops for a moment, reverse them and reclip.
This works slightly differently for big wall climbing.
As the second ascends the fixed rope and cleans the gear, they have a large carabiner hanging from a short sling (usually about 30 cm) from their belay loop. (The short sling is somewhat optional, but it declusters the front of your harness a bit and makes it slightly easier to clip in the knots.) The second ties a backup knot every 30 feet or so and clips it to the carabiner.
This backup knot system is pretty standard practice in big walling, as it does two main things: 1) Protects the cleaner from a huge fall in the event their ascenders become detached from the rope, and 2) Helps prevent the rope from blowing / swinging sideways and getting hung up somewhere where you don't want it to be.
The trick is when you get to the top, you keep all of the loops tied into the carabiner, and transfer that onto the anchor. Ta dah, instant rope management for the lead rope, everything is already tied up in short 15 foot long loops, ready to belay the leader. No rope bag needed. And, as mentioned above, if you are leading in blocks, you unclip the loops for a moment and reverse them, so the correct knot is on top for whoever is leading the next pitch.
For big wall climbing, this is how the harness of the second would look when cleaning and tying the back up knots.