Yakima Railgrab Towers are one component of a car rack for vehicles with flush or raised roof rails. The towers clamp onto your rails with the help of "claws". The adjustable claws work with the towers to create a solid 3-contact point clamping to the roof rail. The claws come in three different sizes:
Small claws for factory rails with height less then 1-1/16".
Medium claws for factory rails with heights between 1-1/16" to 1-3/4".
Large claws are for factory rails with heights between 1-3/4" to 2-1/2".
Add a pair of Yakima round cross bars (sold separately), and you have a complete base roof rack system. Manufacturer's part number: 8000138
The trick I learned after an hour of frustration is that you need to have opposing-side towers connected to a crossbar, and THEN the clamps will grab and hold the roof rails (at least on my 2014 Cherokee Trailhawk). Instructions are to match the clamp positions on all of the towers, but the clunky ratchet system makes that very difficult. It's a lot of trial and error. Once on, they hold tightly.