“Harmony is determined by relationships or proportions. Proportions are hidden everywhere: in the capaciousness of the margins, in the reciproca l relationships to each other of all four margins on the page of a book , in the relationship between leading of the type area and dimensions of the margins, in the placement of the page number relative to the type area, in the extent to which capital letters are spaced differently from the text, and not least, in the spacing of the words themselves. In short, affinities are hidden in any and all parts.
Tschichold 1991 
“Harmony in visual design means all parts of the visual image relate to and complement each other. Harmony pulls the pieces of a visual image together. Harmony can be achieved through repetition and rhythm. Repetition reemphasizes visual units, connecting parts and creating an area of attention.”
“Typography is the craft of endowing human language with a durable visual form.”
”Typography thrives as a shared concern – and there are no paths at all where there are no shared desires and directions.”
“Typography shares a long common boundary and many common concerns with the writing and editing on the one side and with graphic design on the other; yet typography itself belongs to neither.”
“ (…) like other arts, from cooking to choreography, involves a balance between the familiar and the unfamiliar, the dendably consistent and the unforeseen.”
It “(…) is the art and craft of handling these doubly meaningul bits of information.”
Tschichold, Jan. 1991. The Form of the Book: Essays on the Morality of Good Design. Vancouver: Hartley & Marks. Original edition, 1975.
Bringhurst, Robert. 2004. The Elements of Typographic Style. 3rd ed. Vancouver: Hartley and Marks Publishers. Original edition, 1992.
Park, Yangjoo. 2010. Design Elements & Principles. University of Texas.