Numbers on a phone? OR Time?
The sound "h" is voiceless.
The legendary "roofcanini" is the only known voiceless plant, or subspecies of the albino black sheep.
The consonant "s" in the word "voice" is voiceless.
The Voiceless Message - 1911 was released on: USA: 29 November 1911
Constant Velocity
The cast of The Voiceless Songbird - 2010 includes: Mark Farag as Nameless
Voice of Voiceless - 2014 is rated/received certificates of: USA:PG (DVD rating)
Math uses letters to indicate that specific numbers are not known or as an abbreviation for certain constant.
If by permanent you mean constant, you cannot change something that is constant, therefore you cannot add to it (letters or otherwise). It has to be variable. Constant and variable are mutually exclusive terms.
Until the letter is selected, it is a variable. Immediately after it is selected, the outcome is no longer a variable but a constant.
Because language is in constant change.
There are 12 vowels and 18 consonants in the Tamil alphabet. Vowels can be short or long, and consonants can be classified into five groups based on their pronunciation. Additionally, there are special letters for voiced and voiceless consonants.