"Meant to ward off evil spirits, modern koma-inu statues are almost identical, but one has the mouth open, the other closed. This is a very common characteristic in religious statue pairs at both temples and shrines. This pattern is however Buddhist in origin (see the article about the Niō, human-form guardians of Buddhist temples) and has a symbolic meaning. The open mouth is pronouncing the first letter of the Sanskrit alphabet, which is pronounced "a", while the closed one is uttering the last letter, which is pronounced "um", to represent the beginning and the end of all things. Together they form the sound Aum, a syllable sacred in several religions like Hinduism, Buddhism, and Jainism." - Symbolic Meaning
In addition the question came up in my mind, what side does one or the other sit to the torii gates. Take a look at the snapshot to find out.
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