GSM-7 alphabet
GSM-7 is the 7-bit character set used by default for SMS: it opens up 160 characters per message but leaves out some accented letters.
In brief
GSM-7 is the 7-bit character set used by default for SMS: it opens up 160 characters per message but leaves out some accented letters.
Going deeper
Each GSM-7 sign fits on 7 bits. Since an SMS payload is worth 140 bytes, the calculation lands exactly on 160 characters: that's where the text's historical limit comes from.
This set covers the basic Latin letters, digits, common punctuation and a handful of accents, but not all of them. Slipping in a character absent from the table switches the message to UCS-2 and sharply cuts its capacity, hence the habit of removing accents to stay within a single SMS.
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