Which type of hinge is recommended if it is not to be seen in the application?

The recommended choice in this scenario is the butt hinge. Butt hinges are designed to be mounted on the edge of a door and are typically recessed into the door and the frame, making them less visible when the door is closed. This feature allows for a clean and aesthetically pleasing appearance, especially in applications where the hinge is not meant to be a focal point.

Strap hinges, while they may provide strong support for heavier doors, are usually more conspicuous since they extend outward from the edge of the door. Piano hinges, designed for long doors or continuous applications, can also be visible depending on installation. Spring hinges are used for self-closing doors and can be visible as well, which may not align with the requirement of being hidden. Thus, butt hinges are ideal for applications needing discretion and an unobtrusive look.