θ ξ ρ δ

From Wikipedia, the free encyclopedia:

Theta (uppercase Θ, lowercase θ or ϑ; Greek: Θήτα) is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth. In the system of Greek numerals it has a value of 9. Theta has also gained a new significance in the north-eastern region of the United States as a symbol for affection, in a similar capacity to the “heart” symbol. http://en.wikipedia.org/wiki/Theta

Xi (uppercase Ξ, lowercase ξ) is the 14th letter of the Greek alphabet. It is pronounced [ksi] in Modern Greek, and generally pronounced /ˈsaɪ/ (UK) or /ˈzaɪ/ (US) in English. In the system of Greek numerals, it has a value of 60. In the system of Roman numerals, it has a value of 11. The Xi is not to be confused with the letter Chi, which takes the form of the Latin letter X. In ancient times, the Western Greek alphabet used it to represent /kʰ/, while it was used to represent /ks/ in other alphabets. As the alphabet was standardized, Xi was decided to be used for /ks/ and Chi for /kʰ/. While having no Latin derivative, the Xi was adopted into the early Cyrillic alphabet, as the letter ksi (Ѯ, ѯ). http://en.wikipedia.org/wiki/Xi

Rho (uppercase Ρ, lowercase ρ or ϱ) is the 17th letter of the Greek alphabet. In the system of Greek numerals it has a value of 100. It is derived from Semitic Rêš "head" (see Resh). Its uppercase form is not to be confused with the Roman letter P. http://en.wikipedia.org/wiki/Rho_%28letter%29

Delta (uppercase Δ, lowercase δ; Greek: Δέλτα [ðelta] Thelta) is the fourth letter of the Greek alphabet. In the system of Greek numerals it has a value of 4. It was derived from the Phoenician letter Dalet . Letters that arose from Delta include the Latin D and the equivalent in the Cyrillic alphabet: Д. http://en.wikipedia.org/wiki/Delta_%28letter%29