Let $u(x,y)$ be a Harmonic function (then it is assumed also that $u$ is differentiable). Then there exists a $v(x,y)$ such that 1;;$f(x,y):=u(x,y)+iv(x,y)$ is holomorphic. Moreover, such a $v$ is unique up to a constant, and these other such $v$ are called conjugates of $v$.