The black line shows you the result of the red and blue waves being added together. This can look confusing but try and get a feel for it. when you show the grid the solid lines represent nodes and the dashed lines the position of the anti-nodes. Key points to remember are listed below.

[b]Stationary [/b]or [b]standing [/b]waves occur when identical waves moving in opposite directions meet. [br]Certain points have no displacement, these are called [b]nodes[/b]. [br]Other points, the [b]anti-nodes[/b] have maximum displacement.[br][br]Node to node distance = half the wavelength [br]Node to anti-node distance = one quarter the wavelength