The term "leading" (pronounced "ledding") is a carryover from the old manual typesetting days when lead (as in the metallic element Pb) wafers were used to insert space between lines of text. Thus line spacing became known as "leading". There are two types of leading: internal and external. Internal leading is the space between the visible bottom of a character and the bottom of the character cell. (Characters with descenders, like "j", have less internal leading than those without them.) External leading is the space between the bottom of the character cells in one row and the top of the character cells in the next row. Typically, whether on a dumb terminal, a terminal emulator, or A-Shell/Windows, the screen is divided up into character cells with no external leading between the rows. (The apparent space between the rows is due to the internal leading within each cell.)

The motivation for increasing the external leading is that most Windows applications tend to have a lot more vertical space between fields than text applications do. (It is quite normal for a text application to have data entry fields on each line, whereas you will rarely see a Windows form in which the edit boxes are so close together.) To help achieve a more spacious look without modifying the program to actually skip rows, you can just add leading. The more leading you add, the shorter the fonts become for a given screen size.

The effect of changing the leading is immediate, so it is easy to experiment with. It is saved along with other settings by the File..Save or File..Save As menu options, and loaded automatically when A-Shell is launched.