When Gemeentemuseum Den Haag opened in 1935 a diamond-shaped painting with four yellow lines on a white background hung in one of the stairwells. Instructions on the back of the painting explained how to hold and hang it, and at what height. But the painting was so progressive that the museum was not sure where to hang it. And so Piet Mondrian’s Composition with Yellow Lines (1933) ended up in the stairwell, along with some abstract paintings by other modern artists. The Mondrian was donated to the museum after a fundraising campaign organised by artist Charley Toorop. This account illustrates how the Gemeentemuseum’s Mondrian collection came into being. Thanks in part to the donation of two private collections – those of Mondrian’s good friend, estate agent Sal Slijper, and of his childhood friend Albert van den Briel – the museum was able to amass the largest collection of Mondrians in the world. Comprising almost 300 items, the collection covers every stage of Mondrian’s impressive artistic development, from the early landscapes he painted in and around Amsterdam and Domburg, via the fundamental abstraction he discovered in Laren and Paris, to the work he produced in New York, where he found the radical new rhythm that defines his final masterpiece, Victory Boogie Woogie (1942-1944). This icon of modern painting is the crowning glory of Gemeentemuseum Den Haag’s collection.