Generally speaking, there are two main forms of curriculum coherence:
Vertical coherence: When a curriculum is vertically aligned or vertically coherent, what students learn in one lesson, course, or grade level prepares them for the next lesson, course, or grade level. Teaching is purposefully structured and logically sequenced so that students are learning the knowledge and skills that will progressively prepare them for more challenging, higher-level work. For a related discussion, see learning progression.
Horizontal coherence: When a curriculum is horizontally aligned or horizontally coherent, what students are learning in one ninth-grade biology course, for example, mirrors what other students are learning in a different ninth-grade biology course. In addition, the assessments, tests, and other methods teachers use to evaluate learning achievement and progress are based on what has actually been taught to students and on the learning standards that the students are expected to meet in a particular course, subject area, or grade level.
Generally speaking, the concept of a coherent curriculum grew out of the recognition that what is taught and learned in schools may not only be misaligned, but—in more extreme circumstances—random, disordered, and potentially detrimental to students. For example, in some schools teachers might decide what gets taught in a course based on personal preference, convenience, past habits, outdated instructional materials, and other factors unrelated to what is appropriate for or in the best interests of students. In addition, curriculum and instructional expectations for teachers might be uneven or nonexistent, which could lead to educational disparities that disadvantage some students. For example, one teacher might cover a lot of material in a given course, and teach it in an engaging way, while a colleague, teaching a similar course, might teach far less content and teach it comparatively poorly (disparities such as these have been well documented in educational research).
For these and other reasons, in recent decades government agencies and education policies, at both the state and federal levels, have either required or encouraged greater standardization in the education of students, with the general goal of improving educational quality and the academic achievement of students. Schools and districts have also embraced more coherent approaches to the design and delivery of learning experiences, either proactively or in response to changes in educational policies and state requirements. The basic rationale is that when educators are working and teaching in concert, and using developmentally appropriate and well-defined learning expectations, students will learn more and leave school better prepared. By exerting more control over the learning process, the reasoning goes, schools, districts, and government agencies will be able to improve educational quality and minimize the factors that have historically produced poor educational results—although whether certain strategies actually produce the desired results remains a source of ongoing debates.