Obligatory register, obligate Lage. The register that is proper for the Urlinie. It serves as a reference point for motions to other registers, and in normal circumstances serves as the register for the ultimate resolution of the Urlinie. Brahms's variations systematically explore a variety of registers, but continue to relate to the obligatory register.
In the Example, taken from Variation 5, the music uses the obligatory register for the opening and closing measures. But the 4 and 3 of the Urlinie in mm. 6 and 7 provide a registral contrast. They occur one octave higher than the obligatory register.