Horizontal décalage is a concept in Jean Piaget's Theory of Cognitive Development and refers to the observation that once a child has the capability to perform a certain task or function they don't know how to immediately apply the concept to other functions or tasks that share the same conceptual ideation. It is a lag in time in being able to understand different tasks that require the same cognitive framework. Horizontal décalage occurs during the concrete operational stage of cognitive development which is around the ages of 7-11. This concept is most often observed using conservation tasks which we master during this stage. Conservation tasks are used to test the ability to understand that some qualities and properties are the same as before when after an object undergoes physical change.
For example, a common conservation task uses water. When water from a normal glass is poured into a taller thinner glass it appears to be more liquid even though it is the same amount.
Another example would be conservation of matter - picture a ball of clay. When the ball of clay is pressed down into a flat sphere children without an understanding of conservation would not understand that there is still the same amount of clay even though its shape is drastically different. Children typically develop conservation of mass at ages 6-7, conservation of weight at ages 9-10, and conservation of weight around age 11. Horizontal décalage means that although a child has attained the general concept of conservation at age 6 they can't translate this concept into similar conservation tasks until they are older.