Motivation for the CTR model. The physical quantities of the TR of geometrical thickness D in which the Lyα line core forms vary much faster along the local TR normal vector ﹩{\boldsymbol{n}}﹩ than along the perpendicular direction. Therefore, we approximate the line-core formation region using an inclined plane-parallel model whose normal vector ﹩{\boldsymbol{n}}﹩ makes an angle θn with respect to the local vertical direction Z. The quantity L is of the order of the local radius of curvature of the TR.