Simplify the following text for a Grade 11 native English-speaking person.
Original: Sigma ( upper case Î # , lower case Ï , lower case in word-final position Ï ; Greek Ï Î Î Î 1\/4 Î ) is the eighteenth letter of the Greek alphabet , and carries the 'S ' sound .
Simplified: Sigma ( uppercase\/lowercase Î # Ï Ï ) , is the letter of the Greek alphabet , used to represent the `` s '' sound in Ancient and Modern Greek .