Why did the veteran prisoners tattoo Elie when he got to the camp?

In the book, Elie and his father are tattooed with numbers by "veteran prisoners."

Elie writes that the number A-7713 is tattooed onto his left arm and that from that moment on, he is no longer addressed by his name. Historical records have confirmed that the Jewish prisoners were all given numbers; the purpose was to dehumanize the prisoners and to remove all personal pride and identity from the minds of the incarcerated. The Nazis had planned to destroy both the bodies and minds of their prisoners, and they were successful in many millions of cases.

The Nazis had hoped that, by obliterating the personal identities of their prisoners, they would cease to think of themselves as human. To the Nazis, it was also a convenient way to rationalize their immense cruelty towards the prisoners themselves.

According to historical records, only prisoners selected for work at Auschwitz were given tattoos. Those who were sent straight to the crematoriums were not given tattoos or numbers. Originally, numbers were sewn onto uniforms, but due to the high mortality rate among prisoners, metal stamps were used to stamp serial numbers onto a prisoner's left upper chest. When that proved impractical, needles were used to pierce numbers onto the left upper forearms of prisoners.

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