The division of educational systems into different tracks--academic and vocational--represents one of the key elements in explaining social stratification and inequalities. Previous research identifies teachers' expectations as a critical factor to understand the relationship between tracking and social inequality. This paper discusses how ability is represented in teachers' discourses and whether and to what extent it works as a legitimation of systemic forms of tracking. Using in-depth interviews with 35 secondary school tutors, we analyse how teachers draw on the concept of ability to explain students' unequal transitions from a lower comprehensive to an upper tracked education system in Barcelona (Spain). The results indicate three main elements: a highly naturalistic conception of students' abilities among teachers; a remarkably dichotomised conception of theoretical and practical abilities that match with the academic and vocational tracks; and a direct association between types of student and types of track based on different types of ability at a cognitive, behavioural and personal level. Overall, the analysis contributes to opening the 'black box' of the notion of ability as represented by teachers and to identifying what we call the 'mechanisms of misrecognition' which serve to naturalise, legitimise and reproduce a highly segmented post-16 school system.