Horizontal décalage: when children start to know that the mass of an object remains the same whether it is rolled up into a ball or rolled out into a long sausage shape, then Piaget would say that they have understood that its mass remains invariant over transformations in its shape. Now, you and I also know that its weight will remain the same over these transformations in shape too. And we can appreciate that the structural characteristics of our knowledge about these two invariant properties is equivalent: in both cases, X (mass) and Y (weight) remain constant despite changes in Z (shape). But children generally come to understand the invariance of mass about two years before they understand the invariance of weight. The developmental changes that their two understandings go through follow the same course. And in both these cases, the notion of invariance is being worked out at a symbolic level: that is, it's knowledge of the same sort in both situations. And thus Piaget describes it as 'horizontal décalage.