the simplest closed core shape. Windings may be put on one or both legs of the core. "E" core E-shaped core are more symmetric solutions to form a closed magnetic system. Most of the time, the electric circuit is wound around the center leg, whose section area is twice that of each individual outer leg. In 3-phase transformer cores, the legs are of equal size, and all three legs are wound. {{multiple image | align = center | direction = horizontal | width = 200 | image1 = E_core.png | caption1 = Classical E core | image2 = EFD_core.png | caption2 = The EFD core allows for construction of inductors or transformers with a lower profile | image3 = ER_core.png | caption3 = The ETD core has a cylindrical central leg. | image4 = EP_core.png | caption4 = The EP core is halfway between a E and a pot core }} "E" and "I" core Sheets of suitable iron stamped out in shapes like the (sans-serif) letters "E" and "I", are stacked with the "I" against the open end of the "E" to form a 3-legged structure. Coils can be wound around any leg, but usually the center leg is used. This type of core is frequently used for power transformers, autotransformers, and inductors. Pair of "E" cores Again used for iron cores. Similar to using an "E" and "I" together, a pair of "E" cores will accommodate a larger coil former and can produce a larger inductor or transformer. If an air gap is required, the centre leg of the "E" is shortened so that the air gap sits in the middle of the coil to minimize fringing and reduce electromagnetic interference. Planar core A planar core consists of two flat pieces of magnetic material, one above and one below the coil. It is typically used with a flat coil that is part of a printed circuit board. This design is excellent for mass production and allows a high power, small volume transformer to be constructed for low cost. It is not as ideal as either a pot core or toroidal core''' but costs less to produce. Pot core Usually ferrite or similar. This is used for inductors and transformers. The shape of a pot core is round with an internal hollow that almost completely encloses the coil. Usually a pot core is made in two halves which fit