There are some obsolete 1-15R or 1-20R (120V, 15 or 20A) receptacles which are mechanically able to accept 1-15P (120V), 1-20P (120V), 2-15P (220V), or 2-20P (220V) plugs. These receptacles are typically found in older residential buildings and are not allowed to be installed under current NEC codes. In addition to the lack of grounding, these obsolete receptacles could allow a connected device to overheat and create a fire hazard if a device designed for the wrong voltage is connected. These problematic outlets are easily identifiable due to their lack of ground, along with both openings on the receptacle face being a sideways "T" shaped opening that appear to be mirrored on the vertical center line of the face. Due to the potential danger of a voltage mismatch, whenever possible these receptacles should be replaced. Depending on local code, replacement with a "repair" approved non-grounded receptacle may be sufficient.