Unconfined yield strength is a property that influences many processes that handle bulk powder materials. Understanding which particle scale properties affect strength will help engineers design better products prior to production, reducing costly mistakes and increasing productivity. This paper examines the relationship between bulk unconfined yield strength and particle shape. It is an experimental work that suggests that the number of particle contacts per adjacent particle and the direction of these contacts are key parameters influencing the bulk strength. This paper suggests a way that shape effects can be incorporated in predictive models relating strength to particle scale parameters. (C) 2009 Elsevier B.V. All rights reserved.