The neatest references, when many are required in books, are either superior letters or superior figures,—thus, ¹, ², ³, or thus, ᵃ, ᵇ, ᶜ. Superior letters are used chiefly in Bibles and other books which have more than one sort of notes, and therefore require different references. When thus used, the letter ʲ should be omitted, as, from its similarity to the ⁱ, the reader might at times be led into error. ACCENTED LETTERS. Letters called accented by printers are the five vowels, marked thus:— Acute á é í ó ú Grave à è ì ò ù Circumflex â ê î ô û Diæresis ä ë ï ö ü Long ā ē ī ō ū Short ă ĕ ĭ ŏ ŭ We may include the French ç, the Spanish ñ, the Portuguese ã and õ, the Swedish and Norwegian å and ö, and the Welsh ŵ and ŷ. NUMERAL LETTERS. The Greeks at first employed the letters of the entire alphabet to express the first twenty-four numbers; but the system was cumbrous, and they adopted the happy expedient of dividing their alphabet into three portions, using the first to symbolize the 9 digits, the second the 9 tens, and the third the 9 hundreds; and, as their alphabet contained only twenty-four letters, they invented three additional symbols. Their list of symbols then stood as follows:— +-----------------+----------------------+---------------------+ | Units. | Tens. | Hundreds. | +-----------------+----------------------+---------------------+ |α represents 1|ι represents 10|ρ represents 100| |β 2|κ 20|σ 200| |γ 3|λ 30|τ 300| |δ 4|μ 40|υ 400| |ε 5|ν 50|φ 500| |ϝ (introduced) 6|ξ 60|χ 600| |ζ 7|ο 70|ψ 700| |η 8|π 80|ω 800| |θ or ϑ 9|Ϟ or ϟ (introduced) 90|ϡ, [symb], [symb] | | | | (introd’d) 900| +-----------------+----------------------+---------------------+ By these symbols, only numbers under 1000 could be expressed; but, by putting a mark called iota under any symbol, its value was increased a thousand-fold: thus, ᾳ = 1000, κͅ = 20,000; or, by subscribing the letter Μ, the value of a symbol was raised ten thousand-fold. For these two marks, single and double dots were afterward substituted. This improvement enabled them to express with facility all numbers as high as 9,990,000,—a range sufficient for all ordinary purposes.