A trimming apparatus for trimming two working layers, including bonded abrasive applied on mutually facing sides of an upper and a lower working disk of a grinding apparatus configured for simultaneous double-side processing of flat workpieces includes a trimming disk, a plurality of trimming bodies and an outer toothing, where the trimming bodies are configured to release abrasive substances upon contract with the working layers so as to effect material removal from the working layers. At least 80% of the area of the trimming bodies configured to come into contact with the working layers is arranged within a ring-shaped region on the trimming disk. The width of the ring-shaped region is between 1-25% of the diameter of the trimming disk and the area of the trimming bodies which comes into contact with the working layers occupies 20-90% of the total area of the ring-shaped region.