  Liquid crystals in two dimensions do not support long-ranged nematic order,
but a quasi-nematic phase where the orientational correlations decay
algebraically is possible. The transition from the isotropic to the
quasi-nematic phase can be continuous of the Kosterlitz-Thouless type, or it
can be first-order. We report here on a liquid crystal model where the nature
of the isotropic to quasi-nematic transition can be tuned via a single
parameter $p$ in the pair potential. For $p<p_t$, the transition is of the
Kosterlitz-Thouless type, while for $p>p_t$ it is first-order. Precisely at
$p=p_t$, there is a tricritical point, where, in addition to the orientational
correlations, also the positional correlations decay algebraically. The
tricritical behavior is analyzed in detail, including an accurate estimate of
$p_t$. The results follow from extensive Monte Carlo simulations combined with
a finite-size scaling analysis. Paramount in the analysis is a scheme to
facilitate the extrapolation of simulation data in parameters that are not
necessarily field variables (in this case the parameter $p$) the details of
which are also provided. This scheme provides a simple and powerful alternative
for situations where standard histogram reweighting cannot be applied.
