  We study the minimal crossing number $c(K_{1}\# K_{2})$ of composite knots
$K_{1}\# K_{2}$, where $K_1$ and $K_2$ are prime, by relating it to the minimal
crossing number of spatial graphs, in particular the $2n$-theta curve
$\theta_{K_{1},K_{2}}^n$ that results from tying $n$ of the edges of the planar
embedding of the $2n$-theta graph into $K_1$ and the remaining $n$ edges into
$K_2$. We prove that for large enough $n$ we have
$c(\theta_{K_1,K_2}^n)=n(c(K_1)+c(K_2))$. We also formulate additional
relations between the crossing numbers of certain spatial graphs that, if
satisfied, imply the additivity of the crossing number or at least give a lower
bound for $c(K_1\# K_2)$.
