If you don't know sign language, you might not realize that it's far more than translating spoken or written words. Sign language has its own grammar and rules, and it employs more than just hands. And it's different in different countries. See the picture?
This is the sign for the word “math” in two different sign languages—American Sign Language on the left, and Japanese Sign Language on the right. Why should there be more than one sign language? Doesn’t that just complicate things? This question would make sense if sign language was a system invented and then handed over to the deaf community as an assistive device. But sign languages, like spoken languages, developed naturally out of groups of people interacting with each other. We know this because we have observed it happen in real time.
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(Image source: YouTube)