Under what conditions and to what extent do external officeholders in parliamentary democracies constrain the cabinet's freedom of action? The article argues that we must analyse both institutional powers and officeholders’ incentives to use them to obtain an unbiased estimate of the expected constraint. It measures the incentives dimension via the selection method of external officeholders and develops an index to capture the likelihood that such officeholders hold preferences deviant from those of the cabinet. Analysing original data on four external constraint institutions in 25 European democracies, the article shows major variation in the incentives to constrain the cabinet across both offices and countries. Furthermore, it demonstrates that institutional powers and incentives for their use are empirically largely independent dimensions.