  The $g$-fan of a finite dimensional algebra is a fan in its real Grothendieck
group defined by tilting theory. We give a classification of complete $g$-fans
of rank 2. More explicitly, our first main result asserts that every complete
sign-coherent fan of rank 2 is a $g$-fan of some finite dimensional algebra.
Our proof is based on three fundamental results, Gluing Theorem, Rotation
Theorem and Subdivision Theorem, which realize basic operations on fans in the
level of finite dimensional algebras. For each of 16 convex sign-coherent fans
$\Sigma$ of rank 2, our second main result gives a characterization of algebras
$A$ of rank 2 satisfying $\Sigma(A)=\Sigma$. As a by-product of our method, we
prove that for each positive integer $N$, there exists a finite dimensional
algebra $A$ of rank 2 such that the Hasse quiver of the poset of 2-term silting
complexes of $A$ has precisely $N$ connected components.
