Simplify the following text for a Grade 8 native English-speaking person.
Original: Stigma ( uppercase\/lowercase Ï Ï ) , is a sign that was used in the Greek alphabet for the combination of the two letters Sigma ( s ) and Tau ( t ) .
Simplified: Stigma ( Ï ) is a ligature of the Greek letters lunate sigma ( Ï ) and tau ( Î ) .