Prettyprint algorithms designed for printing programs on paper are not appropriate in an interactive environment where the interface to the user is a CRT screen. We describe a data representation and an algorithm that allow the efficient generation of program displays from a parsed internal representation of a program. The displays show the structure of the program by consistent and automatic indentation. They show the program in varying levels of detail by replacing unimportant parts with ellipsis marks. The relative importance of program parts is determined jointly by the structure of the program and by the current focus of attention of the programmer. © 1981, ACM. All rights reserved.