A system of numerals represented by letters of the Greek alphabet. These are used for special purposes, where in English Roman numerals might be used. The numerals are formed by the addition of a mark to the upper or lower case letter, i.e. Γ΄, δ΄, θ΄. In addition to the usual Greek letters three additional symbols are used: stigma (ϛ), koppa (ϟ) and sampi (ϡ); stigma (Ϛ or ϛ) may also be represented by ΣΤ΄ or στ΄.