Dummy-heads are often used for standardized measurements where modeling of the human head and torso is relevant. Monaural Head-Related Transfer Functions of a Brüel & Kjaer dummy were measured in the horizontal plane in one-degree spatial resolution. Evaluation is made by plotting the peak-valley structure in frequency. Special frequency and spatial domains can be determined based on the variations of the HRTFs that are relevant to understand physical properties of the dummy -head in measurements and processes of human directional hearing. Symmetries and similarities of measured HRTFs help to scrutinize the perception of directional information in the monaural and binaural evaluation, the "noisy domain" in frequency and space where shadowing of the head occur as well as the filtering effect of the pinna.