robin
Autoleading is a convenient way of applying the same percentage of leading to all type throughout a document. When you want to make type look denser or more spaced out you set a specific leading value, as the others have described.
The concepts of double spacing and single spacing are pretty meaningless in the context of desktop publishing. They hark back to typewriters when you only had mechanical control of multiples of lines.
Leading goes back to early typesetting days when type was set as single characters or single lines on chunks of soft metal and these chunks were stacked on top of each other to make a column of type. The thickness of the metal dictated the space between the lines, and the term "leading" has remained to describe the spacing of lines of type.
In a DTP program leading can be pretty well anything you like, within the overall capabilities of the program to render it.
Word processors still refer to line spacing because their ancestry is in typewriters not typesetters, and the people who use them would understand line spacing, but are likely to be confused by "leading".
k
k
The problem with auto leading values is they are figured for "average" body text situations, which really don't exist, and may not be appropriate for any particular typeface at any particular size. Some faces have larger x-heights, some short x-heights and long ascenders, and so forth. 120% leading (the default for automatic) may not give optimum spacing between lines to maintain readability (that's the point of leading) in body text, and it is almost always too much in headlines.
Most professional typesetters will specify an absolute leading value for any particular font and size when they define the paragraph styles. This also allows you to set up baseline grids more easily if you need to maintain horizontal alignments across a page or spread.
Peter