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 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. The second part of the model specifies four quality in use characteristics. The characteristics are applicable to every kind of software, including computer programs and data confined in firmware.