In typography, the space between lines of type, also called leading (pronounced "ledding"), vertical spacing, or film advance. The term leading dates back to metal typesetting, when thin strips of lead were inserted by hand between lines (i.e., lines were leaded). In linecasting, the line was one unit of metal called a slug. The slug could be cast with an amount of leading "built-in" or added by hand later. If no leading was present at all, the lines were said to be set solid (for example, 10/10).
Leading should be in proportion to line length and point size—about 20% of the point size—or it should be slightly larger than the optimum word space. Very fine spacing was called carding in hot metal, because pieces of card paper, instead of lead, were inserted; trouble resulted when liquid (i.e., type wash) hit paper slices: columns would suddenly "grow" (expand in depth).
One of the capabilities that modern typesetting techniques make available is called minus leading. This means that the type is set with a leading value less than the point size, for example, 9 on 8H. Usually, this can be done only with faces that are small on body (small x-height), have short ascenders and/or descenders, or for all caps.
Small x-height faces and some sans serifs should have minimal or minus leading. Large x-heights and bold type need more leading. To calculate the minimum amount of leading required between two type lines when changing point sizes, take one-third of the present point size and add it to two-thirds of the point size to be used on the next line. If the leading is not properly set, the lines could overlap one another.
The most important point to remember is that all leading is measured from baseline to baseline. In hot metal, it was merely the incremental space between slugs. Today, it is the total space between lines, which is defined by the baseline.