Caption: Interference of 2 1-dimensional transverse waves (i.e., transverse waveforms or wave states or fields) at the same place in space.

Features:

1. The shown waves---which are all at the same place in space---obey the superposition principle: i.e., the wave strengths of multiple waveforms at each point add linearly.

   Actually, nonlinear interference and diffraction occur too and are common for strong waves. However, for electromagnetic radiation (EMR), the superposition principle applies in almost all usual systems.

2. Exact alignment of waveforms is called "in phase" and exact disalignment of waveforms is called "out of phase."

3. Phase difference is measured in degrees: in phase is 0° phase difference; out of phase is 180° phase difference.

4. Constructive interference is when the waveforms add up to create a larger magnitude waveform. Constructive interference is maximum for in phase waveforms as illustrated in the left panel of the image where the lower two waveforms add to give the upper one.

5. Destructive interference is when the waveforms add up to create a smaller magnitude waveform. Destructive interference is maximum for out of phase waveforms as illustrated in the right panel of the image where the lower two waveforms add to give the upper one (which is zero everywhere).

   Waveforms of the same shape and size out of phase give exact cancellation: complete destructive interference as illustrated in the right panel of the image.

6. Interference and diffraction (which is just interference for multitudinous or a continuum of waveforms) are universal behaviors of actual physical waves---or so yours truly believes---maybe there are some exotic exceptions.