Skinniest rap line??

While much is quantifiable (breaking strength, weight, etc.) resistance to cutting is not. More than once I've had to swallow hard while rapping with thin cords over sharp edges, careful to not let the rope roll. Some rock also tends to generally fracture sharper and have more sharp edges (shale, limestone) than others (gneiss, granite) which may factor into purchasing decision. Rope-life expectancy can be pretty short - no surprise.

I use a 7.7mm twin for rapping, ascending, and occasional leading.

Munter hitch is key for rapping single strand.  You can use 2 in series if you're heavy.

My son uses a 6mm Maxim Esprit Personal Escape Rope (for rapping with 2 ropes, or as a tag line).
It also works in a mini-Traxion.
It could be used to rap single strand if in some emergency.

Kirtis Courkamp wrote: The rad line looks sick untell you see the price tag. 460 $ for fancy accessory cord ...  

Some of them (like the mammut and I am guessing also the grivel one) are crazy expensive but the rad line is less then $460 (for the 60m), at least in Europe. I think i payed around $120 for the 30m. I also recently bought a 50m 5mm rap line from Decathlon for $100 which I am eager to try out. 

Really happy with it for solo tours where I just carry it in the pack for 90% of the tours without ever using it. Also works great for glacier crossings and when skitouring.

I've been using the Edelrid Rap Line 6mm a bunch of times, tied to a 9.1mm lead line and rapping of both strands (to do double rope rappels).
No problems at all with friction in this scenario, it's hardly noticeable that the strands are a different diameter.

Scott Bennett wrote: I've thought a fair amount about this. I really like combining big runs with moderate solos, and sometimes I wanna have a lightweight escape/rap option.

I've used 3mm dyneema. Plenty strong, but hard to get enough friction without generating a lot of heat. Worked, but I wouldn't recommend.

Much better: I use the 5mm Beal Backup line (Dyneema core with nylon sheath, 1200kg Min. Breaking strength, 21g/m) in conjunction with the Beal Escaper (100g). I use the Petzl Pirana rappel device, and it provides great friction and heat management, even when rapping on a single strand. I've got it in 30m, 40m and 70m lengths. Highly recommended.

the 3mm line on the Palisades traverse

What you think about controlling the rappel with the Pirana versus lighter options (Monster Munter, etc.) I also noticed that this is actually a 5.5mm line according to Weighmyrack, but can't be labeled in .5 mm increments. 

5.5 mm tech cord. 5000 lb breaking strength above 900 degree melting point. They also make a 3mm, which was originally developed for military special forces. Use an SMC Escape 8 or the mini 8 from Rock Exotica (has side horn). Also highly recommend the KONG Robot device. If rappelling small diameter ropes you can also use a regular 8 with double wrap or invert it by threading the smaller opening and clipping the large opening.

I personally would avoid the spectra or dyne a, as they have a lower melting point (about 100 degrees lower than nylon!) and is extremely slippery. 

The Tech Cord has a technora core, which is an aramid, and can self abrade if run over a small bend radius repeatly.

I have a 60m RAD line (luckily much cheaper in Europe) that I occasionally use as a tag line. It's obviously super light, but it kinks quite easily, making it quite frustrating to use. Most times I'd prefer to simply climb with my Mammut Twilight 7.5 as twin ropes and just avoid the clusters on the way down. Twlight 38+38g/m vs say Infinity 9.5 which is 59g + 22g. Pros and cons of each setup of course.