SIS
Paul was not crucified upside down, he was beheaded. Peter was the apostle that was, as he said he did not deserve the same death as Jesus.
A circle or a square.
The next palindromic number after 1991 is 2002. A palindromic number reads the same forwards and backwards. In this case, 2002 reads the same whether you read it left to right or right to left.
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The word "NOON" is an example of an image that reads the same right side up and upside down.
Something that looks the same upside down has point symmetry.
1961
NOON
NOON
MOW
Some types of ambigrams read the same from different perspectives.
The word is NOON.
NOON
This is an ambigram.
SWIMS
The word SWIMS, when written in upper case letters, is the longest word in the English language that reads the same both forward and upside-down with no reading difficulty.