Mathematics teachers (K-12) utilize textbooks to largely determine the scope and the sequence of mathematics concepts taught in their classrooms (see, e.g., Braswell et al., 2001; Clements, 2002; Grouws & Smith, 2000; Grouws, Smith, & Sztajn, 2004; Woodward & Elliot, 1990). However, little is known of the nature of the learning trajectories of important mathematical concepts defined by textbook authors in the written curricula that serve as the conceptual basis for the scope and sequence of what and when mathematics is taught. Olson (2010) identified articulated learning trajectories (ALTs) defined by authors’ placement of concepts within the written text. The ALTs identified were related to the development of algebraic thinking concepts (e.g., functions) through the use of patterning concepts within four middle school mathematics textbook series: Saxon Math (Saxon) (Hake, 2007), Glencoe Mathematics: Applications and Concepts (Glencoe) (Bailey et al., 2006), McDougal Littell Math Thematics (Math Thematics) (Billstein & Williamson, 2008), and Connected Mathematics 2 (CMP) (Lappan, Fey, Fitzgerald, Friel, & Phillips, 2009). Importantly, differences in the development of algebraic concepts were identified among the four textbook series, as was the divergent use of mathematics terms critical in the mathematical development identified in the ALTs examined in the four curricula.