question: Why do two-pronged plugs in the US almost always only fit one way?
comment: Some devices require the electricity to flow in a certajn way. The POLARITY of the current is important in these cases. This detail of plugs help thi EDIT: FYI one is positive and the other is neutral
passages: In Brazil, this kind of plug is commonly found in high-power appliances like air conditioners, dishwashers, and household ovens. The reasons why they have been unofficially adopted for this use may be the robustness and high-current bearing capabilities, the impossibility of inverting phase (active) and neutral pins, or the fact that Argentina, a border country, uses this plug and used to be more developed than Brazil in the past so there may have been a flux of high-powered appliances from Argentina to Brazil at some point in time.
   With the introduction of the first electric cars in the UK they were initially able to connect to the electricity grid with using a 3-pin domestic socket or a "Type 1" connection as it was developed in the USA as SAE J1772. The ACEA (organisation of European automobile makers) however settled on the "Type 2" connector that allows access to the 3-phase power grid. The UK Department for Transport agreed with the position and funding for charging stations favours the Type 2 connector. Many charging point manufacturers are beginning to include a "Type 2 Mode 3" (Mennekes type) socket in addition to a domestic 3-pin socket on their products.
   When BS 546 was in common use domestically in the UK the standard did not require sockets to be shuttered, although many were. The current revision of the standard allows optional shutters similar to those of . Current UK wiring regulations require socket outlets installed in homes to be shuttered.
  
  Section::::British three-pin (round) plugs and sockets.:BS 546.
   May 1950: BS 1363:1947 Amendment 3, title changed to "Specification for two-pole and earthing-pin fused-plugs and shuttered socket-outlets for A.C. circuits up to 250 Volts (not intended for use on D.C. circuits)".
  
  January 1957: BS 1363:1947 Amendment 5, added clause permitting operation of shutters by simultaneous insertion of two or more pins (in addition to original method using only earth pin).
  
  January 1957: BS 1363:1947 Supplement No. 1 added specification for surface mounted socket-outlets.
   The Plugs and Sockets, etc. (Safety) Regulations 1994 permit domestic appliances fitted with non-BS 1363 plugs to be supplied in the UK with conversion plugs fitted, but not with conversion plugs supplied for fitting by the consumer.
  
  Section::::BS 1363 three-pin (rectangular) plugs and sockets.:BS 1363 variations.
  
  Section::::BS 1363 three-pin (rectangular) plugs and sockets.:BS 1363 variations.:Folding plugs.
   Section::::Obsolete types.
  
  Section::::Obsolete types.:Old Spanish sockets.
  
  Some older industrial buildings in Spain used sockets that took a plug rated for higher current and had two flat contacts and a round earth pin, somewhat similar in design to the ones found on American plugs but larger in size. The two flat contacts are spaced further apart than on an American plug. No domestic appliances were ever sold with these plugs. 
  
  The line and neutral measure , and are apart. All three pins are long, and the earth pin is a cylinder of diameter.
  
  Section::::Obsolete types.:Original American Hubbell plug and receptacle.
   In North America, there are two types of pin and sleeve products that are used:
  
  Section::::North America.:Pin and sleeve.:NEMA.
   Section::::BS 1363 three-pin (rectangular) plugs and sockets.:BS 1363-1 "Rewirable and non-rewirable 13 A fused plugs".:Fuses.
   CEE 7/1 unearthed sockets accept CEE 7/2 round plugs with pins. Because they have no earth connections they have been or are being phased out in most countries. Some countries still permit their use in dry areas. Older sockets are so shallow that it is possible to accidentally touch the live pins of a plug. CEE 7/1 sockets also accept CEE 7/4, CEE 7/6 and CEE 7/7 plugs without providing an earth connection. The earthed CEE 7/3 and CEE 7/5 sockets do not allow insertion of CEE 7/2 unearthed round plugs.
  
  Section::::Types in present use.:CEE 7 standard.:CEE 7/3 socket and CEE 7/4 plug (German "Schuko"; Type F).
   To reduce the risk of users accidentally touching energized conductors and thereby experiencing electric shock, plug and socket systems often incorporate safety features in addition to the recessed slots or holes of the energized socket. These may include plugs with insulated sleeves, recessed sockets, sockets with blocking shutters, and sockets designed to accept only compatible plugs inserted in the correct orientation.
  
  The term "plug" is in general and technical use in all forms of English, common alternatives being "power plug", 
  
  "electric plug",
  
  and (in the UK) "plug top".
   The plug and socket are rated 16 A 250 V a.c. and are intended for use only on systems having nominal voltages between 200 V and 250 V a.c. The plug pins are 4.5 mm in diameter, line and neutral are on centres 19 mm apart. The earth pin is offset 3.0 mm. The line pin is on the left when looking at a socket with the earth pin offset down. Shutters over the line and neutral pins are mandatory.
  
  The only country to have officially adopted the standard is South Africa as SANS 164-2.
   Section::::BS 1363 three-pin (rectangular) plugs and sockets.:BS 1363-1 "Rewirable and non-rewirable 13 A fused plugs".:Ratings.
   Few private houses in Brazil have an earthed supply, so even if a three-pin socket is present it is not safe to assume that all three terminals are actually connected. Most large domestic appliances were sold with the option to fit a flying earth tail to be locally earthed, but many consumers were unsure how to use this and so didn't connect it. The new standard has an earth pin, which in theory eliminates the need for the flying earth tail.
  
  Section::::Types in present use.:British and compatible standards.
   A variation (standard DK 2-5a) of the Danish plug is for use only on surge protected computer sockets. It fits into the corresponding computer socket and the normal socket, but normal plugs deliberately don't fit into the special computer socket. The plug is often used in companies, but rarely in private homes.
  
  There is a variation for hospital equipment with a rectangular left pin, which is used for life support equipment.
   The parallel and tandem socket accepts normal parallel NEMA 1‐15 plugs and also tandem NEMA 2‐15 plugs. Both pairs of sockets are fed internally by the same supply.
   A more recent and fairly common version of this type is the T-slot socket, in which the locations of the tandem and the parallel slots were combined to create T-shaped slots. This version also accepts normal parallel NEMA 1‐15 plugs and also tandem NEMA 2‐15 plugs. Incidentally, a NEMA 5‐20, NEMA 6-15, or NEMA 6‐20 plug with a missing earth pin would fit this socket. This receptacle type has been unavailable in retail shops since the 1960s but still available from the manufacturer Leviton (model 5000-I) for replacement only and not for new installations.
   BULLET::::- Distributors allowed an output pulse to be wired to more than one input without creating a back circuit between the inputs.
   Section::::BS 1363 three-pin (rectangular) plugs and sockets.:BS 1363-1 "Rewirable and non-rewirable 13 A fused plugs".:Counterfeits and non-standard plugs.
   The German Schuko-system plug is believed to date from 1925 and is attributed to Albert Büttner. As the need for safer installations became apparent, earthed three-contact systems were made mandatory in most industrial countries.
  
  Section::::Development.:Proliferation.
   Standard plugs and sockets based on two round pins with centres spaced at 19 mm are in use in Europe, most of which are listed in IEC/TR 60083 "Plugs and socket-outlets for domestic and similar general use standardized in member countries of IEC". EU countries each have their own regulations and national standards; for example, some require child-resistant shutters, while others do not. CE marking is neither applicable nor permitted on plugs and sockets.
  
  Section::::Types in present use.:CEE 7 standard.:CEE 7/1 unearthed socket and CEE 7/2 unearthed plug.
   The two top pins are the “live” pins. The top right plug pin is the live or active pin while the top left plug pin is the neutral pin. The third pin, if provided, is the earth or ground pin. The live pins of every 10 amp and 15 amp flat pin plug sold after 3 April 2005 have been required to be insulated, in accordance with AS/NZS 3112:2000.
   When looking at the front of the socket with the earth aperture uppermost (as normally mounted) the lower left aperture is for the neutral contact, and the lower right is for the line contact.
  
  Section::::BS 1363 three-pin (rectangular) plugs and sockets.:BS 1363-2 "13 A switched and unswitched socket-outlets".:Shutters.
   Type I plugs and sockets from different countries have different pin lengths. This means that the uninsulated pins of a Chinese plug may become live while there is still a large enough gap between the faces of the plug and socket to allow a finger to touch the pin.
  
  Section::::Types in present use.:Danish Section 107-2-D1 earthed (Type K).
  
  This Danish standard plug is described in the Danish Plug Equipment Section 107-2-D1 Standard sheet (SRAF1962/DB 16/87 DN10A-R). The Danish standard provides for sockets to have child-resistant shutters.
   Section::::British three-pin (round) plugs and sockets.
  
  In the early 20th century, A. P. Lundberg & Sons of London manufactured the "Tripin" earthed plug available in 2.5 A and 5 A models. The "Tripin" is described in a 1911 book dealing with the electrical products of A. P. Lundberg & Sons and its pin configuration appears virtually identical to modern BS 546 plugs.
   The earliest domestic plug and socket is believed to be that patented by T.T. Smith in 1883. This was shortly followed by patents from W.B. Sayers and G. Hookham; these early designs had rectangular plugs with contact plates on either side. In 1885, two-pin plug designs appeared and in 1889 there were two-pin plugs and sockets in the GEC catalogue. The 1893 GEC Catalogue included 3 sizes of what was described as "Double plug Sockets" with capacities described not in amps, but as "1 to 5 lights", "5 to 10 lights" and "10 to 20 lights". These were clearly recognisable as two-pin plugs and sockets, but with no indication as to pin size or spacing, they were sold as pairs. The same catalogue included lampholder plugs for both BC and ES lampholders (capacity unspecified), and also a type of two-pole concentric plug and socket (similar to a very large version of the concentric connectors used for laptop PC power connections) in the "1 to 5 lights" and "5 to 10 lights" capacities. Crompton and Company introduced the first two-pin socket with protective shutters in 1893, and the Edison Swan Company was also manufacturing two-pin plug and sockets in the 1890s.
  
raw_presuppositions: Two-pronged plugs in the US almost always only fit one way
raw_corrections: Some devices fit one way, others do not.