Using a long slinky and oscillating one end at the correct frequency, standing waves can be formed. The first is called the fundamental or 1st harmonic. The next oscillation is called the 1st overtone or 2nd harmonic and so on.
The video demonstrates the 1st, 2nd and 3rd harmonics.
The world record hand-generated standing wave is the 7th harmonic !!!
This video gives an explanation how standing waves are generated.