These can be called either standing waves or stationary waves, both names are used for the same thing. You might find it easier to watch the video at the bottom of this page first.

If two lots of waves meet they will interfere with each other. If the wavelength and the frequency are the same, if they have a similar amplitude and they occur at the same time then this is called coherence. A recognisable interference pattern will be formed.

Interference

Where the two waves meet crest to crest or trough to trough, then we get constructive interference - a large wave is formed.

If the waves meet trough to crest then they cancel out completely. This is destructive interference.

If we shake a rope which is fixed at one end, then the waves travel down the rope and reflect from the fixed end. The reflected wave then meets the wave going down and the two interfere. If the wavelength of the waves is a simple fraction or multiple of the length of the rope then the interference pattern is fixed in one place. The waves reflect up and down the rope easily. This is a condition known as resonance and the interference pattern, which looks fixed, is called a standing wave.

We can create standing waves in a large number of situations. The two that are the most common examples in student examinations and practical work are vibrations in a string/rope and sound in a tube. There are more detailed notes with supporting videos here:

Standing waves in wires

Standing waves in tubes

These are the simplified versions of the conditions that exist in most musical instruments.

Note: These diagrams are a graphical representation of the amplitude of the vibration of air molecules in the tube. The direction of vibration is up and down the tube.