all, same
咸 centers on the idea of completeness or uniformity: everything together, all, or the same throughout.
咸 is thought to combine 戌 (a halberd-like weapon) and 口 (mouth), possibly representing a unified command or all speaking together, but the exact historical development is uncertain.
Imagine a halberd (戌) and a mouth (口) together: a single voice commanding all troops, making everyone the same and united.
For カン, picture a can containing everything: all the same items packed together. The can cues カン and the kanji's meaning of 'all, same'.
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