Parametrized surfaces of low degrees are very useful in applications,
specially in Computer Aided Geometric Design and Geometric
Modeling. The precise description of their
geometry is not easy in general. Here we study surfaces of bidegree
$(1,2)$. We show that, generically up to linear changes of coordinates,
they are classified by two continuous parameters (modulus). We present
an elegant combinatorial description where these modulus appear as
cross ratios. We
provide compact implicit equations for these surfaces and for their singular
locus together with a geometric interpretation.
