Stigma (letter)

Stigma (uppercase/lowercase Ϛ ϛ), is a sign that was used in the Greek alphabet for the combination of the two letters Sigma (σ) and Tau (τ). It is also used as a symbol for the number 6 in the system of Greek numerals.

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Greek alphabet
Αα Alpha Νν Nu
Ββ Beta Ξξ Xi
Γγ Gamma Οο Omicron
Δδ Delta Ππ Pi
Εε Epsilon Ρρ Rho
Ζζ Zeta Σσς Sigma
Ηη Eta Ττ Tau
Θθ Theta Υυ Upsilon
Ιι Iota Φφ Phi
Κκ Kappa Χχ Chi
Λλ Lambda Ψψ Psi
Μμ Mu Ωω Omega
Other letters
Ϝϝ Digamma Ⱶⱶ Heta
Ϻϻ San Ϙϙ Koppa
Ϡϡ Sampi Ϛϛ Stigma
Ϸϸ Sho