This pair of stackable header (also called Arduino stackable header) can be soldered to a board or simply used as a connector that you can be easily plugged and unplugged. This product comes in pairs.

If you need to solder them, make sure your header pins are not crooked. Stack up a shield on your Arduino board while you are soldering to make sure your headers are properly aligned. Please notice that this model is the standard Arduino header (as compared to the extra long legs model that can also be found).