Surly's Monkey Nuts provide extra tire and/or derailleur clearance by moving the axle back in the dropouts. Monkey Nut V2 is compatible with most 2016 or older Surly models with rear-exiting horizontal dropouts and 10mm quick-release or solid axle hubs such as the Troll, Ogre, ECR, Karate Monkey or Krampus. The two-piece design clamps into the front of your dropouts and moves your wheel 14mm farther back.
These are not compatible with the 2017 and newer "Gnot-Boost" thru-axle dropouts. You'll want a Snuggnut chain tensioner or the Monkey Nut V3 for that application.