# Isothermal net

An orthogonal net on a surface $V^2$ in Euclidean $3$-space in which the small quadrangles formed by two pairs of lines from distinct families are, up to infinitesimal quantities of the first order, squares. The lines of an isothermal net are level curves of two conjugate harmonic functions. In the parameters of an isothermal net the line element has the form:

$$ds^2=\lambda^2(du^2+dv^2),$$

where $\lambda=\lambda(u,v)$. An isothermal net is a particular case of a rhombic net. On a surface of rotation the meridians and parallels form an isothermal net; an asymptotic net on a minimal surface is isothermal.