What is the difference between 260x240 and 240x260?
The distinction is purely a matter of orientation. In 240x260, the width is 240 cm and the length is 260 cm. In 260x240, the two values are reversed: 260 cm wide by 240 cm long.
In practice, a 240x260 cover and a 260x240 cover fit the same 240x260 cm duvet. The choice depends on the drop you want on each side of the mattress. A width of 260 cm offers a more generous drop, which is practical when two people sleep together.