Double-unders are an inconsistant skill for me. I feel as if I sometimes take them into the gym with me and sometimes leave them in the car. It's not uncommon for me to only manage to connect 2 or 3 together in a set. Last Friday, I did a number of sets of that size. I also did several sets of more than 30 and a set of 67. That 67 was a personal record. Clearly, I need to practice more.