# Why does the first peak of the temperature power spectrum of CMB imply a flat Universe?

According to Wikipedia, the first peak of the temperature power spectrum of CMB determines the curvature of the Universe. And this answer by @pela says that the first peak is consistent with a flat Universe. So my questions are how/why does the first peak tell us the curvature?

The fact that the first peak is expected near $$l=200$$ if the curvature is zero comes from a calculation of the size of the horizon at recombination. One finds that the horizon at that time has an angular size of about 0.9 degrees of arc in the sky today, corresponding to $$l=180/0.9=200$$.