LEGO introduced its System in Play concept in 1955.
The idea connected separate LEGO sets into one coherent construction system. Instead of treating each box as an isolated toy, the system allowed houses, vehicles, streets, and other models to work together. This gave children a reason to add new sets while keeping older bricks useful.
The concept was promoted by Godtfred Kirk Christiansen, who emphasized that LEGO products should support creative building across a complete play environment. Earlier automatic-binding bricks had already established the basic interlocking principle, but System in Play expanded that principle into a planned product philosophy.
People sometimes confuse 1955 with 1958, the year associated with LEGO’s modern tube-and-stud brick design patent. The dates describe different milestones: System in Play was a product-system concept, while 1958 concerned the geometry that made the modern brick especially stable and reusable.