Ability grouping refers to the sorting of students into different classes or groups based on their ability in a curricular area. In elementary school, within-class ability grouping is common where students are organised into one of several groups in their own classroom for a maths or reading lesson. In middle and high school, between-class ability grouping, or tracking, is common; students enrol in different classes based on their prior achievements in a curricular area. Other relatively common types of ability grouping include cross-grade subject grouping, the Joplin plan, and grouping gifted students together. Proponents of ability grouping argue that these tactics allow teachers to tailor their pace and lessons better for their students, while opponents argue that these grouping practices can accentuate achievement gaps. Thus, the effectiveness of ability grouping appears to vary. Overall, ability grouping should be used minimally, in reading or maths only, to hierarchically build upon the knowledge of students. By maintaining a small number of groups and ensuring that students can move between groups as appropriate, ability grouping can be effectively implemented in classrooms.