  Most 1d hydrodynamic models of plasma confined to magnetic flux tubes assume
circular cross-section of these tubes. We use potential field models to show
that flux tubes in circumstances relevant to the solar corona do not in general
maintain the same cross-sectional shape through their length and therefore the
assumption of a circular cross-section is rarely true. We support our
hypothesis with mathematical reasoning and numeric experiments. We demonstrate
that lifting this assumption in realistic non-circular loops make apparent
expansion of magnetic flux tubes consistent with that of observed coronal
loops. We propose that in a bundle of ribbon-like loops those that are viewed
along the wide direction would stand out against those that are viewed across
the wide direction, due to the difference in their column depths. That would
impose a bias towards selecting loops that appear not to be expanding seen
projected in the plane of sky. An implication of this selection bias is that
the preferentially selected non-circular loops would appear to have increased
pressure scale height even if they are resolved by current instruments.
