Plain - plane

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These are two homophones that can be confused in spelling by some writers. (They are both pronounced IPA: /pleɪn/.)

Plane and plain form one of the sets of homophones listed by the then Poet Laureate Robert Bridges.
(For more, see Bridges homophones). AWE has a category listing our articles on each of these.

  • Plain is most usually an adjective, meaning 'without any embellishment', or 'simple'. 'The plain truth' means 'the simple unvarnished truth'. (This phrase is sometimes used in discussion to conceal the fact that the subject is more complicated than the speaker can deal with: academics should always be aware of Oscar Wilde's "The truth is rarely pure and never simple" (The Importance of Being Earnest, 1895, Act I).) When this adjective is applied to people, it is never a compliment. It means that they are not good-looking. When applied to decoration, art or ornament, it may be a compliment, suggesting honesty, straightforwardness and an avoidance of the over-complicated.
    • The noun 'a plain' labels a geographical feature: a level area of land of many possible sizes. An 'alluvial plain' is a flat area formed by a river flooding and depositing silt in a fairly uniform sheet; a 'coastal plain' is an area of flat land formed by the sea, either by erosion or deposition. Examples of large plains include the Great Plains of central North America, the pampas of South America, the Steppes of Russia and the Veldt of South Africa.
  • A plane is
    • in geometry and related subjects 'a flat surface', the equivalent in two dimensions of a line in one dimension. A plane may be a term for the imaginary way a body may be seen, as the human body may be viewed from the anterior, posterior, inferior, and lateral planes. In Optics, there are several planes such as the plane of incidence and the plane of refraction.
      • The related adjective plane is used in Geometry to describe a perfectly flat or level surface, with no curvature.
      • In Geography, Engineering etc an inclined plane, which may not be perfectly flat, is a slope. A ramp is an example.
    • Figuratively, a plane may be a metaphorical level, most often in spiritual or intellectual contexts. A problem may be on the mental or the physical plane; spiritualists talk of the astral plane, where the spirits live. We can say that an artist like Michelangelo elevated sculpture to a new plane.
    • More literally, in forms of transport, a plane is a flat (or subtly curved) surface used to alter the flow of air, or water. The wings of aircraft in this sense are planes (commonly now called 'airfoils'), and submarines dive or surface by the use of hydroplanes, 'a kind of rudder in the vertical direction'. This used to be called a hydroplane: this term is now only used for a sort of 'boat' that travels by skimming over the surface of the water, rather than going through it (commercial vessels on the same principle are usually called 'hydrofoils'). Both aeroplanes and hydroplanes use the principle of 'lift' to carry their loads.
      • Perhaps most commonly, a plane is, informally, an abbreviated form of 'aeroplane' ('airplane' in the USA)
    • a tool used usually by carpenters to level down a smooth wooden surface by removing thin curls of wood. There is a similar tool used to produce the same effect on a surface of soft metal.
    • A plane tree is a tree belonging to the genus platanus, which includes the sycamore. In Britain the most common variety is probably the 'London plane', Platanus X hispanica.
      • The verb 'to plane' is connected with one or other of
        • The carpenter's tool. A woodworker may plane a piece of wood to make it flat, or the edge of a door to make it close;
        • The 'foil': an aircraft without power may plane (or glide) safely to the ground; a racing sailing dinghy sometimes accelerates greatly when the sailor can make it 'plane', or skim over, the water, rather than going through it.

The confusion that sometimes arises between these two homophones is usually connected with the nouns used (plain) in Geography and (plane) in geometry and engineering. Both have the general meaning of 'a flat and level area'. The latter is more precisely flat, and is more of an intellectual concept, the latter a label for a realler phenomenon. Curiously, both are derived from the same original - the Latin plānum, 'flat or level ground'.

You may also want to see about another confusion of spellings, at planed and planned; and there is an article at Plain sailing.