  We develop a magnetic ribbon model for molecular cloud filaments. These
result from turbulent compression in a molecular cloud in which the background
magnetic field sets a preferred direction. We argue that this is a natural
model for filaments and is based on the interplay between turbulence, strong
magnetic fields, and gravitationally-driven ambipolar diffusion, rather than
pure gravity and thermal pressure. An analytic model for the formation of
magnetic ribbons that is based on numerical simulations is used to derive a
lateral width of a magnetic ribbon. This differs from the thickness along the
magnetic field direction, which is essentially the Jeans scale. We use our
model to calculate a synthetic observed relation between apparent width in
projection versus observed column density. The relationship is relatively flat,
similar to observations, and unlike the simple expectation based on a Jeans
length argument.
