The HP-18C is a Hewlett-Packard business calculator which was quickly followed by the very similar but greatly improved HP-19B.
The HP-18C is HP's first RPL-based calculator internally,[1] even though this was not visible on user-level in this non user-programmable model.
The user has a solver (another HP first) available, but only had about 1.5 KB of continuous memory available to store equations.
The calculator has many functions buried in a menu structure.
The clamshell design is fairly robust, but the battery door is the shortcoming of this whole line; 18C, 19B, and 28C/S models.