CLASP observations of the intensity (top panel), Q/I (middle panel), and U/I (bottom panel) line-center signals of the hydrogen Lyα line. Each point corresponds to a pixel along the slit of the spectrograph with its corresponding value for the cosine of the heliocentic angle, μ. The reference direction for the positive Stokes Q is the parallel to the nearest limb. The dashed curve in the middle panel shows the CLV of the Q/I line-center signal calculated by Trujillo Bueno et al. (2011) in the semi-empirical model C of Fontenla et al. (1993), while the dotted curve shows the CLV of the ﹩\langle Q\rangle /\langle I\rangle ﹩ line-center signal calculated by Štěpán et al. (2015) in the 3D MHD model of Carlsson et al. (2016; with the symbol ﹩\langle ...\rangle ﹩ meaning spatial average at each μ position). We note that the standard deviation of the observed Q/I and U/I signals due to the presence of noise is σ = 0.05%, i.e., comparable to the vertical size of the data markers in the plot.