These notes, it will be seen, compose two octaves of the Diatonic scale, identical with the two octaves of the Greater Perfect System. They may be regarded as answering to the white notes of the modern keyboard,--those which form the complete scale in the so-called 'natural' key. The other notes, viz. those which are required not only in different keys of the Diatonic scale, but also in all Enharmonic and Chromatic scales, are represented by the same characters modified in some simple way. Usually a character is turned half round backwards to raise it by one small interval (as from Hypatê to Parhypatê), and reversed to raise it by both (Hypatê to Lichanos). Thus the letter epsilon, [Symbols: E], stands for our _c_: and accordingly [Symbols: W] ([Symbols: E] [Greek: anestrammenon] or [Greek: hyption]) stands for _c*_, and [Symbols: 3] ([Symbols: E] [Greek: apestrammenon]) for _c[Symbols: #]_. The Enharmonic scale _c-c*-c[Symbols: #]-f_ is therefore written [Symbols: E W 3 f'], the two modifications of the letter [Symbols: E] representing the two 'moveable' notes of the tetrachord. Similarly we have the triads [Symbols: h I rl, F "q, Cup, KY>1, <V>, CUm]. As some letters do not admit of this kind of differentiation, other methods are employed. Thus [Symbols: D] is made to yield the forms [Symbols: ri] (for [Symbols: 7]) [Symbols: L A]: from [Symbols: H] (or [Symbols: B]) are obtained the forms [Symbols: Li] and [Symbols: R]: and from [Symbols: Z] (or [Symbols: i]) the forms [Symbols: A] and [Symbols: A]. The modifications of [Symbols: N] are [Symbols: /] and [Symbols: \]: those of [Symbols: 'I] are [Symbols: A] and [Symbols: N]. The method of writing a Chromatic tetrachord is the same, except that the higher of the two moveable notes is marked by a bar or accent. Thus the tetrachord _c c[Symbols: #] d f_ is written [Symbols: E W 3' /`']. In the Diatonic genus we should have expected that the original characters would have been used for the tetrachords _b c d e_ and _e f g a_; and that in other tetrachords the second note, being a semitone above the first, would have been represented by a reversed letter ([Greek: gramma apestrammenon]). In fact, however, the Diatonic Parhypatê and Tritê are written with the same character as the Enharmonic. That is to say, the tetrachord _b c d e_ is not written [Symbols: h E H r], but [Symbols: Fix I-r]: and _d e[Symbols: b] f g_ is not [Symbols: I], but [Symbols: I-tl F]. Let us now consider how this scheme of symbols is related to the Systems already described and the Keys in which those Systems may be set ([Greek: tonoi eph' hôn tithemena ta systêmata melôdeitai]).