Sheshach - supposed to be equivalent to Babel (Babylon

Babylon

Babylon was an Akkadian city-state of ancient Mesopotamia, the remains of which are found in present-day Al Hillah, Babil Province, Iraq, about 85 kilometers south of Baghdad...

),
according to a secret mode of writing

Writing

Writing is the representation of language in a textual medium through the use of a set of signs or symbols . It is distinguished from illustration, such as cave drawing and painting, and non-symbolic preservation of language via non-textual media, such as magnetic tape audio.Writing most likely...

 among the
Jews

Jews

The Jews , also known as the Jewish people, are a nation and ethnoreligious group originating in the Israelites or Hebrews of the Ancient Near East. The Jewish ethnicity, nationality, and religion are strongly interrelated, as Judaism is the traditional faith of the Jewish nation...

 of unknown antiquity

Ancient history

Ancient history is the study of the written past from the beginning of recorded human history to the Early Middle Ages. The span of recorded history is roughly 5,000 years, with Cuneiform script, the oldest discovered form of coherent writing, from the protoliterate period around the 30th century BC...

, which consisted in substituting the
last letter

Letter (alphabet)

A letter is a grapheme in an alphabetic system of writing, such as the Greek alphabet and its descendants. Letters compose phonemes and each phoneme represents a phone in the spoken form of the language....

 of the Hebrew alphabet

Hebrew alphabet

The Hebrew alphabet , known variously by scholars as the Jewish script, square script, block script, or more historically, the Assyrian script, is used in the writing of the Hebrew language, as well as other Jewish languages, most notably Yiddish, Ladino, and Judeo-Arabic. There have been two...

 for the first, the last but
one for the second, and so on. Thus the letters sh, sh, ch become b, b, l, i.e., Babel (see: Atbash cipher).This code is called the Atbash Cipher and was used to decrypt. The city does not appear on any map nor in any other documents, save the book of Jeremiah. The Atbash Cipher was applied and out of Sheshach came the word Babel. This is supposed to be confirmed by
a reference to Jer. 51:41, where Sheshach and Babylon are in
parallel clauses. There seems to be no reason to doubt that
Babylon is here intended by this name. (See Streane's Jeremiah,
l.c.)