At age 8, Case proposed, children have sufficiently mastered this central conceptual structure that they can begin to use two number lines simultaneously to solve mathematical problems. For example, they can now answer such questions as, “Which number is bigger, 32 or 28?” and “Which number is closer to 25, 21 or 18?” Such questions require them to compare digits in both the “ones” column and “tens” column, with each comparison taking place along a separate mental number line. In addition, 8-year-lods presumably have a better understanding of operations that require transformations across columns, such as “carrying a 1” to the tens column during addition or “borrowing a 1” from the tens column during subtraction.