Allen style nuts for modern bikes. If you have a modernish frame (made within the last 29.7 years) you probably need the Allen, as opposed to bolt on. The rear has 28mm of exposed bolt, the front has 50mm of exposed bolt.
These exist because we wanted a sidepull that would accommodate a fat tire and fender and make it easy to release the wheel. Tektro agreed to it, and these are them. The brake reach range is 55mm to 73mm, which exceeds by far anything Shimano or Campy has, which makes them good for country and touring bikes. They're lighter than Ultegra sidepulls, stop just as well, and are beautifully made in all respects.
There is no box, no instructions, no cables or housing. You get two calipers, friend, and then you're on your own.
Popular for 700c-to-650B conversions.
On this new and improved R559 model, all of the calipers come with a new feature. It is neither bad nor good. It is as neutral as a feature gets. It is this: above the quick-release lever on the caliper arm, is a quick-release lock, which prevents the quick-release lever from opening unless you disengage the lock, which you do by simply pressing the tiny button while pulling up on the q/r lever. Adding this feature has the unfortunate consequence of making the pre-feature model seem not as good, which is nuts. But, in these lititgidguolous times, it is probably prudent.
Sold as a set (front and rear).
Buy the 54mm length koolstop insert pads after you've done a lot of riding, I mean braking.