Hermannus Contactus, a monk in the Swiss abbey of Reichenau, proposed that the six-note chord, C-A sums up, with greatest possible economy, the "sound" of Gregorian chant. Therefore the "finals" (see last post), where extended downwards to include the C. The tetrachords beginning on those finals (now C, D, E, and F) now begin to point to what we now think of as major and minor tonality.
The Italian monk, Guido of Arezzo, proposed, around 1030, placing nuemes on the lines and spaces of a ruled staff to define their precise pitch. He used colours, and then clefs, to denote "key lines"; points of reference to be used to find the pitches of the surrounding notes.
A specialist in ear-training, Guido chanced upon a tune (the hymn Ut queant laxis) that not only contained each diatonic interval up to and including a fifth, but where each half-line began on each of the notes proposed by Harmannus. The syllables at these points (corresponding with C, D, E, F, G, and A) are Ut, re, Mi, fa, Sol, and la. What we now know as sol-fege.
The full major scale came about when singers in the early seventeenth century began adding an extra note (si, later becoming ti) at the top of Hermannus's hexachord. Guido achieved something similar by transposing and superimposing the hexachord back onto itself, thus creating the gamut: a full range of pitches that were later represented on the Guidonian hand. Placing this pitches on the hand was a technique derived from public speakers and the like, and acted as a mnemonic aid. It is also where such expressions as "At one's finger-tips" or "rule of thumb" come from (the thumb on Guido's hand contains the first, most basic note of the gamut).