Helical pipes and ducts are used widely in industry. There is a huge literature on flow in curved pipes and bends, but comparatively little has been published on flow in helical pipes. To simplify the problem, most theoretical work on flow in helical pipes has assumed zero pitch: a toroidal pipe following the work of Dean in 1927. Recent theoretical studies on helical pipes have used helical coordinate systems. These systems, however, are not appropriate when studying helical ducts. The following results are reported: a coordinate system appropriate for helical ducts; the detailed equations of continuity and motion in this coordinate system; the numerical method used to find the fully developed laminar flow of a Newtonian liquid in helical ducts; the simulations vs. published experimental data; and effects of the pitch ratio, curvature ratio and pressure gradient on secondary flow in a helical duct.