“…Cobb, Wood, & Yackel, 1993;Cobb & Bauersfeld, 1996) did use models of tens and ones (often unifix cubes stored in columns of ten), but few teachers seem to have used systematic activities with such models to facilitate all children's construction of generative tens conceptual structures. Some children in third grade (Lo, Wheatley, & Smith, 1994) and in the fourth grade (Steinberg, Carpenter, & Fennema, 1994) were still using unitary methods. In our experience, all second graders, even those coming from backgrounds of urban poverty, can come to carry out and explain generalisable addition and subtraction methods involving tens by several months into the school year if they have experiences to help them construct the conceptual prerequisites for such methods (Fuson, 1996;Fuson & Smith, 1995;.…”