Kappa (uppercase Κ, lowercase κ or ϰ; Greek: Κάππα) is the 10th letter of the Greek alphabet, used to represent the voiceless velar stop, or "k", sound in Ancient and Modern Greek. In the system of Greek numerals it has a value of 20. It was derived from the Phoenician letter Kaph. Letters that arose from kappa include the Roman K and Cyrillic К. Greek proper names and placenames containing kappa are often written in English with "c" due to contemporary rendering into the Latin alphabet by ancient Romans. However, modern transliteration of the Greek language invariably use the letter "k". Compare Κυνοσκεφαλαι, Cynoscephalae (ancient placename); κυνος κεφαλαι, kunos kephalai (phrase), meaning "dog's heads". In mathematics, the kappa curve is named after this letter; the tangents of this curve were first calculated by Barrow (17th Century). In graph theory, the connectivity of a graph is given by κ. In differential geometry, the curvature of a curve is given by κ. In physics, the torsional constant of an oscillator is given by κ. In chemistry, the compressibility of a compound is given by κ. In physics, the Einstein constant of gravitation In psychology and psychiatry, kappa represents a measure of diagnostic reliability. In biology, kappa and kappa prime are important nucleotide motifs for a tertiary interaction of group II introns. In biology, kappa designates a subtype of an antibody component. In pharmacology, kappa represents a type of opioid receptors. Kappa statistics such as Cohen's kappa and Fleiss' kappa are methods for calculating inter-rater agreement. Upper-case letter Κ is used as a symbol for: In textual criticism, the Byzantine text-type (from Κοινη, Koine, the common text). In set theory, kappa is often used to denote an ordinal which is also a cardinal.