Handheld augmented reality (HAR) applications must be carefully designed and improved based on user feedback to sustain commercial use. However, no standard questionnaire considers perceptual and ergonomic issues found in HAR. We address this issue by creating a HAR Usability Scale (HARUS). To create HARUS, we performed a systematic literature review to enumerate user-reported issues in HAR applications. Based on these issues, we created a questionnaire measuring manipulability - the ease of handling the HAR system, and comprehensibility - the ease of understanding the information presented by HAR. We then provide evidences of validity and reliability of the HARUS questionnaire by applying it to three experiments. The results show that HARUS consistently correlates with other subjective and objective measures of usability, thereby supporting its concurrent validity. Moreover, HARUS obtained a good Cronbach's alpha in all three experiments, thereby demonstrating internally consistency. HARUS, as well as its decomposition into individual manipulability and comprehensibility scores, are evaluation tools that researchers and professionals can use to analyze their HAR applications. By providing such a tool, they can gain quality feedback from users to improve their HAR applications towards commercial success.