 The tactile momentary switch. It's the button backbone of PCB user interfaces. Available in a variety of form factors, tactile switches are probably most recognizable as the ever-present reset switch on a microcontroller board. But you probably don't realize how many of your devices actually use them, as they're often hidden behind a front panel. Tactile switches are convenient, but wiring them up can be confusing. Because it's easier to forget which pins are connected which way, was it these two are connected to each other? If you don't have a multimeter handy, just solder to posts at opposite corners. Those will always be disconnected until the switch is pressed. Easy.