A model of human performance on intraline text editing, the modification of a line of text, is presented. This model is based on an optimal minimum keystroke solution to the text modification plus a description of those keystrokes that are not easily predicted: feedback, command selection idiosyncrasies, and errors. To better match the human, who is assumed to employ an optimal sequence of editing commands, the optimal solution-was constrained to use the same types of command each human had been observed to use. The model's constraints and descriptions of keystrokes both give rise to an increased understanding of text editing and recommendations for editor design improvements.