Panel a) is the histogram of the vertical height of 121 H ii regions (including 109 UCH ii regions) with respect to the Galactic plane, the half width at half maximum is 25.4  ±  7.1 pc. Panel b) is the histogram of the vertical height of 157 H ii regions from Boston university H ii region catalog, the half width at half maximum is 28.6  ±  8.7 pc. Positive height corresponds to positive Galactic latitude. The overlaid curve shows the Gaussian fit to the data.