A developable surface is a smooth surface whose Gaussian curvature vanishes everywhere. A ruled surface is a surface where for each point there must be a line passing through the point lying on the surface. See https://en.wikipedia.org/wiki/Developable_surface and https://en.wikipedia.org/wiki/Ruled_surface for both definitions. I know that a ruled surface need not be a developable surface, say a hyperboloid for example. However, I'm not able to show that all developable surfaces must be ruled.
It is a folklore that any developable surface must be ruled, and there are two references so far I know. One is Pogorelov's text book "exterior geometry of convex surfaces", but unfortunately I cannot get access to it. The other one is Hartman and Nirenberg's 1959 paper https://www.jstor.org/stable/2372995?seq=1#metadata_info_tab_contents, but in that paper they proved much more. I personally care about an answer which gives a simple understanding of the folklore without going into too many technical details.
I'm not sure if my intuition is correct. The following is how I visualize in my mind: If a surface $z=u(x,y)$ has vanishing Gaussian curvature everywhere, then the Hessian of $u$ is degenerate and has nontrivial kernels. In other words, the second fundamental form must be the squared of a $1$-form, say $II=\left(f(x,y)dx+g(x,y)dy\right)^2$. Hence along the curve defined by $dy/dx=-f(x,y)/g(x,y)$ the second fundamental form vanishes and this means the normal to the surface is constant along the lifting of the above curve in the surface. Then the only possibility is that the curve must be a line.
I don't have much geometry background. I wonder is the above intuition correct? Or can anyone give me a readable reference? Any comments or ideas would be really appreciated.