In order to reach the Taiyuin Mausoleum from Nikko, you have to pass through three gates. The first one, called Niomon Gate, is “guarded” by two Nio guardians, the two Deva Kings. One of them is represented with the mouth open, vocalizing “A” - the pronunciation of the first grapheme of Sanskrit Devanāgarī (अ), while the other one is represented with the mouth closed, vocalizing an “U” - the last grapheme (म) is pronounced “hūṃ". Together they symbolize the fact that everything has a beginning and an end.
Here’s a detail with my favorite, Naraen Kongō (or Agyō), the guardian with the open mouth:
Yesterday’s Japan Photo: