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Greek letters-as-numbers used an older Greek alphabet with three more otherwise unused letters, two of them re‑instated in their old locations, early in the alphabet. So positions higher than 5th place (ε) were shifted from the standard alphabet; 5th place was marked with normal fifth letter epsilon (ε). The 6th letter in the conventional alphabet, that normally follows ε is ζ (zeta) but the number 6 was represented a revived ancient letter ′ϝ (digamma), followed by ′ζ which was pushed up from 6th to its ancient position (7th) to represent the number 7. All of the letters after ζ were likewise shifted up one place, until the second ancient letter koppa, (ϙ), was reached; it fell between π and ρ. Ever letter from ρ to ω was shifted two places past its conventional ordinal position. Last place coming right after omega (ω, 800) was sampi (ϡ) which represented 900. (From that point, the system restarted, with a new tick-mark, at ͵α. The tick-mark was put in a different place (͵α rather than ′α) to show that the letter represented a multiple of 1,000 rather than 1.)
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From Euclid up to the 19th century, mathematical and technical diagrams were habitually marked sequentially with letters (or numbers), whereas in modern mathematical and scientific diagrams, it is much more common to choose for markers letters that might remind readers of the word used to describe the item in question. For example, Feynman diagrams in particle physics label the positions of particles with the first letter of their name, either in the Latin or Greek alphabet. So p, n, and e , represent the position on a diagram of a proton, neutron, and electron, respectively. The neutrino is represented by ν (Greek "nu"), since the Latin letter "n" is reserved for the neutron.
^SexagesimalGreek numbers in the Almagest are conventional: 1 2 3 4 5 6 7 8 9 = ′α ′β ′γ ′δ ′ε ′ϝ ′ζ ′η ′θ . Notice that ancient digamma (ϝ) is used for 6 instead of zeta (ζ, which is used for 7) ; 10 20 30 40 50 = ′ι ′κ ′λ ′μ ′ν . Adjacent number-letters add, so all the other numbers are made by letter pairs, such as 29 30 31 = ′κθ ′λ ′λα . The number 59 (′νθ) is the largest value used in a single cell in sexagesimal. That leaves xi (ξ) and the letters following it ξ ο π ϙ ρ σ τ υ φ χ ψ ω ϡ free for other use: Ptolemy picked ο , which normally was used for 70, to mark empty (zero) cells, perhaps because the word for "nothing", οὐδέν starts with an omicron.
