Subtract multi-digit numbers using the standard algorithm, regrouping across one or more zero columns.
Subtraction sometimes needs to regroup (borrow) from the next column over. That is easy when the next column has something to lend. But when the next column is a zero, it has nothing to give, so the borrow has to travel further left, column by column, until it finds a digit that can lend. That traveling borrow is a regrouping chain.
Once a column further left lends a 1, it turns into 10 for the column right next to it, which can then pass a 1 along, and so on, all the way back to the column that first needed it.
Mastery looks like: They subtract multi-digit numbers accurately, correctly chaining the regrouping through one or more zero columns.
Common stumbles: Stopping the chain at the first zero instead of continuing left; forgetting to reduce the lending column by 1 after it lends; subtracting the smaller digit from the larger one regardless of position instead of regrouping.
See it visually
Interactive, spoken-aloud tools that show this idea in motion.