Let $\Gamma$ be a family of plane curves. The isocline of $\Gamma$ is the locus of the points, in which all members of $\Gamma$ have an equal slope.

If the family $\Gamma$ has the differential equation $$F(x,\,y,\,\frac{dy}{dx}) = 0,$$ then the equation of any isocline of $\Gamma$ has the form $$F(x,\,y,\,K) = 0$$ where $K$ is constant.

For example, the family $$y = e^{Cx}$$ of exponential curves satisfies the differential equation $\frac{dy}{dx} = Ce^{Cx}$ or $\frac{dy}{dx} = Cy$ , whence the isoclines are $Cy =K$ , i.e. they are horizontal lines.

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