Phase difference in a standing wave?

What would the phase difference between P and Q be? I assumed that because they are 1/4 of a wavelength apart, it would be Pi/2, but supposedly the difference is 0.

• The phase difference is zero. – KV18 Apr 22 at 9:22
• @KV18 but why is that? – Olly Scargill Apr 22 at 9:23
• Have a look at this simulation and observe what is happening to the particles between nodes. – Farcher Apr 22 at 9:28

Both $$P$$ and $$Q$$ have the same amplitudes at the same time. That is because the combining waves have no phase difference between them at that point.