The 12-tooth narrow-wide design on these CeramicSpeed pulleys decreases chain articulation and ensures a crisp chain engagement on to and off of each pulley. 
The symetrical 12-tooth narrow-wide design allows the chain to move smoothly on and off the pulleys without additional drag. 
CeramicSpeed Pulley Wheels for Shimano 11s, 12-tooth is developed, designed and handbuilt in Denmark and comes with seperate dust covers to fit 9100 series or R8000 series derailleurs.