For the "De Rotterdam" lobby, project-leading architect Kees van Casteren was looking for a sofa that was equally comfortable, timeless, and classic-looking, as a contrast to the more modern, lighter fabric chairs he had already selected.

He remained true to his principle: "Always choose a piece of furniture that becomes more beautiful with age" – a principle that applies to the entire de Sede collection. Stephan Hürlemann's DS-21 was a perfect fit for the desired flexible layout, which would allow for various uses with fixed and temporary seating, as well as high and low seating. He was also helped by the "always very pleasant collaboration with de Sede."

According to van Casteren, the DS-21 "fits perfectly into the collection of diverse chairs and sofas that can withstand intensive use by large groups while still maintaining the feel of a living room." This symbiosis of comfort, timelessness, modular properties, and the contrast with the light-looking fabric chairs was ultimately what brought the DS-21 into the vertical city.