"The method of geometric modeling of multi-shell roofs depends mostly on the surface's properties forming the shell; their curvature, as well as continuity between them. ...s play a specific role, due to their characteristics. Catalan surfaces are s... They are oblique ruled surfaces which can be divided into two groups... second order—hyperbolic paraboloid... [and] of more than second order—s, cylindroids... The difference between hyperbolic paraboloid, conoid, and cylindroid results from different path of movement of a surface's ruling during formation. In all cases of Catalan surfaces' creation... each ruling is parallel to the fixed plane (not containing the surface's directrices)."