# Two charts with regular peaks and troughs but only one is stationary?

I'm reading Forecasting Principles and Practice section on stationarity. We are shown the following image and asked which are stationary:

Obvious seasonality rules out series (d), (h) and (i). Trends and changing levels rules out series (a), (c), (e), (f) and (i). Increasing variance also rules out (i). That leaves only (b) and (g) as stationary series.

I'm looking at the first two charts on the bottom row, g and h. Both have peaks and troughs.

Why is h considered stationary while g is not?