Many children have difficulty understanding place value, and, as noted earlier, they frequently use buggy procedures that reflect this lack of understanding. For example, second-graders often do not correctly carry when adding multidigit numbers (Fuson & Briars, 1990). Instead, they either write the two-digit sums beneath each column of single-digit addends (e. g., 568+778=121316) or ignore the carried values (e. g., 568+778=1236).