In fact, the lines above the insertion point do not change, while the lines below move down, and one is normally lost at the bottom of the screen. Deleting a line means causing the line to disappear from the screen, closing up the gap by moving the lines below it upward. A new line appears at the bottom of the screen. Insertion and deletion of lines is useful in programs that maintain an updating display some parts of which may get longer or shorter. They are also useful in editors for scrolling parts of the screen, and for redisplaying after lines of text are killed or inserted.
Many terminals provide commands to insert or delete a single line at the cursor position.
Some provide the ability to insert or delete several lines with one command, using the number of lines to insert or delete as a parameter. Always move the cursor to column zero before using any of these commands. The existing line, and all lines below it, are moved down. The cursor must be at the left margin before this command is used. This command does not move the cursor.