This part of ISO 9241 provides recommendations for command dialogues used to accomplish typical office tasks using visual display terminals (VDTs). Command dialogues are sequences of instructions provided by the user to the system which, when processed, result in associated system actions. Users input (from recall, rather than selecting from a menu) complete or abbreviated command phrases (e.g. mnemonics, letters, function keys, hot keys in the order required by the command language syntax and the computer performs the activities initiated by the command(s) and their associated parameters. Interface design depends upon the task, the user, the environment, and the available technology. Consequently, ISO 9241-15 cannot be applied without a knowledge of the design and use context of the interface and it is not intended to be used as a prescriptive set of rules to be applied in their entirety. Rather, it assumes that the designer has proper information available concerning task and user requirements and understands the use of available technology (this may require consultation with a qualified ergonomics professional as well as empirical testing with real users). ISO 9241-15 applies to the use of command dialogues, either in conjunction with other dialogues (e.g. menus, direct manipulation) or as the primary dialogue technique (e.g. in the case of"dumb terminals" or where high speed is required in a particular application). In addition, this part of ISO 9241 provides recommendations for those "key" commands (i.e. function keys and hot keys) which represent commands within a command dialogue. If the command functions are evident from the nature of their representation (e.g. pictorial icons) and invoking these functions does not require memory on the part of the user, this would not be considered a command dialogue according to ISO 9241-15. Commands can be accessed through other dialogue techniques (e.g. menu options, forms, direct manipulation). However, these methods do not require recall on the part of the user and will be excluded from this part of the standard and will be dealt with in other parts. It also should be noted that ISO 9241-15 does not provide guidance for dialogues which use "natural" language.