What is leading?
Leading is the amount of space added between lines of text to make a document legible. Leading originally referred to the thin lead spacers that printers used to physically increase space between lines of metal type. Most applications automatically apply leading based on the point size of the font. Closer leading will fit more text on the page, but will also decrease its legibility. Looser leading spreads text out to fill a page and will make the document easier to read. However, leading can be negative, in which case the lines of text are too close and they will overlap or touch. Use leading responsibly, and don't overdo it. Leading is the amount of space between the baselines of two vertically adjacent lines of type--one line, and the line either right above it or right below it. The term comes from the size of the lead block the letter sits on--a popular type size was "10 on 12," which was a 10-point font on lead blocks 12 points high. In the ancient days when every printer owned his own little type foundry, he could cast his type with the leading he usually used built right in--if you always ran 12 point Bodoni on 16 points of lead, you'd cast 12-point Bodoni on 16 points of lead. Why you'd run anything 12 on 16 I don't know, but if you always used that lead, you'd set your type up for it. When you set up your chase with this 12 on 16 Bodoni, the type blocks would touch and we'd call that "set solid." The little strips amlove referred to were used to expand the leading. You could get them in lead but no one liked the lead ones--lead is easily malleable, which is a problem when you're dealing with something that's about as thick as a sheet of paper. The best ones were made of brass by Hermann Berthold in Berlin, Germany. The "traditional" amount of leading for body copy was two points larger than the type size--10 on 12 was normal, as this gives you six lines per inch. That's usually what you get with "auto" leading on a desktop system. Now! If someone says they have "two points of leading" it means they've got two MORE points of leading in there than the machine normally sets. This would make 10 on 14 instead of 10 on 12. As amlove said, you can also have negative leading, and it's used in display work--tight leading makes bold heads look even bolder. To get negative leading in hot metal, you just choke down the molds. On an electronic system, just feed in what you want.