The document gives the specifications for a twodimensional geometrical model of a linear bridge for the numerical calculation of the linear thermal transmittance of the linear thermal bridge and the lover limit of the minimum surface temperatures. These specifications include the geometrical boundaries and subdivisions of the model, the thermal boundary conditions and the thermal values and relationships to be used. It is based on the following assumptions: steady-state conditions apply, all physical properties are independent of temeprature, there are no heat source within the building component, only one internal environment applies and one or two external thermal environments apply. In case when soil is part the geometrical modell, the temperature at the horizontal cutt-off plane in the soil is the second external thermal environment.