The goal of this paper was to investigate the role of formal constraints (e.g. definitions, theorems) in geometric reasoning. Four students participated in a task-based interview including 2D Euclidean geometric locus problems. Data were obtained from observations, interviews, and video recordings and analyzed by Toulmin's argumentation model. The analysis of the data provided a detailed look at students' abductive, inductive, and deductive reasoning. Results revealed that formal constraints reduced the uncertainty of a claim. Students could transform abduction and induction to deduction if they could reverse the array of formal constraint usage. The co-existence of warrant and backing including formal constraints facilitated this transfer and this co-existence in reversing their usage was the key factor to eliminate the challenge of transforming abduction to deduction.