horizontal prototype – οριζόντιο πρωτόπυπο (see also vertical prototype)
describes what types of capabilities are implemented in a prototype: horizontal prototypes display a wide range of features but without fully implementing all of those features; vertical prototypes do not attempt to show all that will be in a system but instead focus on implementing a small set of features in a nearly-complete fashion.
Horizontal prototypes are appropriate for understanding relationships across a broad system and for showing the range of abilities of a system.
Vertical prototypes are most appropriate when a certain complex feature of a system is poorly-understood and needs to be explored, e.g. as a proof-of-concept.