# Phase difference problem

What's the phase difference between A and B on the following diagram. Where it is a standing wave. This question doesn't even make sense to me as from watching animations of standing waves, points A and B cannot even appear at the same time. We cannot have two minimums next to each other. So what is it asking?

A standing wave, $$s(x,t)$$, is a product of a time periodic function with a spatial periodic function. In 1D this means $$s(x,t) = f(x)(g(t)$$ where $$f(x+p)=f(x)$$ and $$g(t+T)=g(t)$$, some $$p$$ and $$T$$. For example, $$s(t)=A\sin(kx+\psi)\sin(\omega t +\phi)$$. In the product the temporal phase and the spatial phase are not related.
This $$s(t)$$ could represent a pair of boundary conditions, say, fixed "walls" at $$x=0$$ and $$x=L$$ for $$s(0,t)=0$$ and $$s(L,t)=0$$ when $$\sin(\psi)=0$$ and $$\sin(kL+\psi)=0$$, from which follow $$\psi=m\pi$$ and $$kL+\psi =m'\pi$$ and $$k=\frac{n\pi}{L};$$ $$n=m'-m$$ irrespective of the temporal phase and frequency.