In typography the term leading is the amount of vertical space between lines of text. This may sound rather insignificant in the design of an advertisement or brochure etc, but it can make or break the effectiveness of the piece.
The term leading comes from the days when type was set buy hand in printing presses. Each letter was cast on an individual block and placed side by side to make up the line of text to be printed. Lead strips were placed between these lines to add the vertical white space.
When it comes to typography I believe that it has one objective which is communication. This communication breaks down into two parts: Readability and Graphic Style, and leading plays an important role in achieving these objectives.
Readability is reading ease. Text that is easy to read improves the speed at which it is read, communication and comprehension and order of information. Correct use of leading increases readability. If the lines are set too close together it makes it hard for the readers eye to track from one line to the next. If the leading is set with too much vertical space between the lines the reader again finds it difficult to track from one line to the next. (see below).
Graphic Style creates emotional and logical responses from the layout of the page. Leading can be used to create light and shade on a page. Solid leading results in dark areas of text, but if you open up the leading you get a lighter text area. Therefore clever use of leading can be used to create light and dark areas on the page which can help the reader to navigate the page, and create visual interest without having to rely on graphic elements such as illustrations or photographs, because the text itself can become a graphic element.
So come on and open up a little.