Resumen

This study explores the specialised knowledge deployed by a mathematics teacher using the Open Algorithms Based on Numbers (ABN)methodover a series of lessons on subtraction. The theoretical tool used for the analysis is the Mathematics Teachers Specialised Knowledge (MTSK) model. The research took the form of acase study ofa primary teacher working at aCharterSchool in Huelva, Spain. The data consisted of the transcripts of lessons involving a total structured follow-up interview. The transcripts were broken down into units of information, and analysed for evidenceof specialised knowledge within several subdomains of Mathematical Knowledge and Pedagogical ContentKnowledge. The resultsallow us to conjecturea relationshipbetween this knowledgeand the teacher’straining in the ABN method.