Schematic depiction of the innermost parts of the circumstellar disk of a T Tauri star. Brγ emission in T Tauri stars is associated with the concept of magnetospheric accretion, but outflows caused by certain types of disk or stellar winds (dashed lines) can heat the hydrogen gas to the necessary temperatures to produce Brγ emission as well. Different origin mechanisms can be tied to different spatial scales. Magnetospheric accretion flows (depicted in red) can only stably form in a compact region within the co-rotation radius R_co, where the gaseous disk rotates at the same angular velocity γ as the star. Spatially extended Brγ emission from outside R_co cannot be attributed to magnetospheric accretion and must be caused by some other emission component. This figure specifically shows the common case of a non-axisymmetric magnetosphere, featuring a tilt between the stellar rotational axis γ_* and the magnetic dipole µ. The obliquity of the magnetosphere leads to the formation of two accretion columns, which funnel gas from a wide arc at the truncation radius onto one primary hot spot per hemisphere, as first suggested in Bertout et al. (1988) and Calvet & Hartmann (1992). The equations and physical quantities relevant for the determination of the co-rotation radius R_co and the magnetic truncation radius R_tr are discussed in Sect. 5. For the equations used to estimate the sublimation radius R_sub, which indicates the area at which the disk becomes sufficiently hot to destroy dust grains, we refer to GRAVITY Collaboration (2021b).