The abbreviation OS in Schlagwerk's Precise OS and Urban OS cajon ranges is in a class of its own: OS (Open sound and fully Screwed playing surface) is a new, innovative feature in the Schlagwerk cajon universe. The most apparent feature in the OS models is the new, fully screwed playing surface. During development, vibration points were determined and used to find the best positions for the screws so that the innovative screw pattern could be designed. The resulting effect is, on the one hand, an extremely direct snare response. But on the other hand, this special design coupled with the reduced contract surface has created a very open basic sound. Thanks to the convenient design, the additional reinforcing bars on the inner sides boost the strong bass in the OS models, without interfering with the sensitivity of the snare response.