Typography is a powerful tool of communication. Type has a dual purpose, on the one hand it attracts and creates impact through its dynamic form, on the other it must impart critical information with clarity. The designer makes an analysis of the supplied ‘Copy’. The designer then determines how emphasis, structure and hierarchy can be created within text-based information. There are techniques that are commonly employed to encourage people to read information in the order you intend. Emphasis is given through various means such as: contrast in size, weight, position and colour of type (and use of percentage tints). Use of typographic devices such as rules or reversing type white out of a solid can also be considered. Care needs to be taken however, as too many shifts in emphasis will defeat the original purpose of communicating with clarity the given information. Typefaces such as Univers and Helvetica are particularly suitable for gaining emphasis and their considered design lends itself to imparting information with clarity. Idiosyncratic typefaces (the fancy ones) are not so well suited for the purpose of clear communication. Typography is not the selection of a typeface.
The choice of type size for reading is dependant on the particular typeface (some faces look smaller or larger than others at the same point size). The most common type sizes for reading at up to arms length would be between 9-12pt, however, depending on the length of text some designers will employ sizes as small as 7pt. Small type sizes have been considered chic by some graphic designers. On a functional level one should consider the end user/audience. The RNIB have guidelines for minimum type sizes for people with a visual impairment. Poster design requires a different type of decision-making; it is not just an enlarged flyer. Posters should be judged at actual size and from the distance they are likely to be seen. Small type sizes are the most common issue with posters. The two issues to consider are legibility and readability, most typography is legible, but readability is the measure by which one is inclined to read the particular information.
Leading, line space or line feed are all terms for the space between lines. Space is inserted between lines in order to aid readability. A rule of thumb might be that the wider the measure type is being set over the more leading required, and conversely the smaller the measure type is set over the less leading that is needed. The term leading derives from letterpress where lines are spaced using strips of lead.
The measure is the width over which type is set. For normal reading conditions between 7-10 words per line is considered optimum although this can vary either way. One should be aware that long line lengths are tiring to read eg 20 or more words. Short sentences require the reader to change line more frequently, which interrupts the reading flow. Lines of type within short amounts of text are split both for their appearance (to create an even ragged right edge) and for their sense. You need to read the copy in order to make decisions about its appearance.
Typographic contrast is achieved through various means such as a change in weight, size, position, colour, etc. The aim is to create a change in emphasis and therefore the change needs to be perceptible. Some designers feel that the different between Gill Sans Regular and Gill Sans Bold is not particularly significant. If one is using size to gain emphasis it is unlikely that 1pt difference will be perceived, you might consider a more mathematically satisfying relationship eg 9pt and 18pt.