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This part of ISO/IEC 9126 describes a two-part model for software product quality: a) internal quality and external quality, and b) quality in use. The first part of the model specifies six characteristics for internal and external quality, which are further subdivided into subcharacteristics. These subcharacteristics are manifested externally when the software is used as a part of a computer system, and are a result of internal software attributes. This part of ISO/IEC 9126 does not elaborate the model for internal and external quality below the level of subcharacteristics. The second part of the model specifies four quality in use characteristics, but does not elaborate the model for quality in use below the level of characteristics. Quality in use is the combined effect for the user of the six software product quality characteristics. The characteristics defined are applicable to every kind of software, including computer programs and data contained in firmware. The characteristics and subcharacteristics provide consistent terminology for software product quality. They also provide a framework for specifying quality requirements for software, and making trade-offs between software product capabilities. Normative Annex A provides recommendations and requirements for software product metrics and quality in use metrics. Examples of these metrics are contained in other parts of ISO/IEC 9126. These metrics are applicable when specifying the quality requirements and the design goals for software products, including intermediate products. An explanation of how this quality model can be applied in software product evaluation is contained in ISO/IEC 14598-1.