Following the poor sales of the AO-1, Propst and Nelson went back to the drawing board.[1][2] For several years, Propst and Nelson fought over a disagreement on the work environment best suited to the employee of a corporate office, and Nelson was eventually taken off the project.[1][2] Nelson's departure left Propst free to indulge in his concept of an office capable of constant change to suit the changing needs of the employee, without having to purchase new furnishings, and allowing the employee a degree of privacy, and the ability to personalize the work environment without impacting the environment of the workers nearby.[1][2] Propst recognized that people are more productive within a territorial enclave that they can personalize, but also that they require vistas outside their space.[1][2] Propst's concept was the "back-up", a two- or three-sided vertical division that defined territory and afforded privacy without hindering the ability to view or participate in surrounding activities.[1][2]