The origin of a plaque's coordinate system in Plaque Simulator is at the center of a sphere defined by the radius of curvature of the plaque's inner (concave) surface. The Z axis of this coordinate system is also the central axis of the plaque. In the Plaque window you are looking at a 2D projection of the plaque onto the XY plane. The default center of a linear source (eg I-125 seed) or a beta patch source (eg Ru-106) is tangent to a sphere of radius R' = R + Offset, with the Z axis passing through the center of the source at Z = -R'. The orientation of the linear source is parallel to the Y axis (Tilt = 0°). Plaque Simulator then uses the angle Alpha or its projection onto the XY plane, the distance r, and angles Beta and Tilt to displace the source center away from its default location and orientation on the sphere. Linear sources normally remain tangent to the sphere, beta patches follow the curvature of the sphere.