To visualize what happens when transverse waves travel in both directions along the same string and interfere, try the simulation found at:
The simulation is a JavaScript "Applet" showing two traveling transverse waves of equal frequency and amplitude, going in opposite directions in the same string. As the waves overlap (superposition), their amplitudes add up point-by-point to determine the displacement of the string with time. Using your left mouse button, double click on a gray area of the screen to slow down the animation (do this two or three times). Then use your right mouse button with single clicks to freeze/unfreeze the motion; try to make the waves cancel out (destructive interference at the antinodes) and then make them add to their greatest total amplitude (constructive interference at the antinodes). Note that the separation distance between any two successive nodes, or between any two successive antinodes, is always half the wavelength of either traveling wave.