The following equations represent progressive transverse waves ${\mathrm{Z}}_1=\mathrm{Acos}(\mathrm{\omega }\mathrm{t}-\mathrm{kx})$, ${\mathrm{Z}}_2=\mathrm{Acos}(\mathrm{\omega }\mathrm{t}+\mathrm{kx})$, ${\mathrm{Z}}_3=\mathrm{Acos}(\mathrm{\omega }\mathrm{t}+\mathrm{ky})$  and ${\mathrm{Z}}_4=\mathrm{Acos}(2\mathrm{\omega }\mathrm{t}-2\mathrm{ky})$. A stationary wave will be formed by superposing