Abstract: A biaxially stretched polyester base film having many fine protrusions on its surface, characterized by having the following properties:(a) in a distribution curve representing the relation between the number of protrusions (y/mm.sup.2) and the height of protrusions (x in .mu.m) on the surface of said film which are measured in a region having at least 20 protrusions/mm.sup.2, that part of the distribution curve at which the protrusion height (x) is larger than its peak value crosses the line of log.sub.10 y=-18x+3.7,(b) when the number of protrusions (y) on the film surface is at least 20/mm.sup.2, no protrusion exists which has a height (x) of at least 0.13 .mu.m,(c) said film has a surface roughness (Ra) of not more than 0.012 .mu.m,(d) said film has a Young's modulus in the longitudinal direction of at least 650 kg/mm.sup.2, and(e) when heat-treated at 70.degree. C. for 1 hour under no load, said film has a heat shrinkage in the longitudinal direction of not more than 0.1%.

Abstract: A biaxially oriented polyester film for use in thin metallic film-type magnetic recording media, said polyester film having a first surface to which a thin metallic film as a magnetic recording layer is to be applied and a second surface coated with an easily slippable layer composed of an organic polymeric binder and inert fine solid particles dispersed therein, the first surface having (a) large protrusions with a diameter of 0.2 to 2 micrometers and a height of 20 to 200 angstrom distributed at a density of at least 10.sup.3 but less than 10.sup.6 per mm.sup.2, and (b) small protrusions with a diameter of 0.01 to 0.1 micrometer and a height of 10 to 100 angstrom distributed at a density of from 10.sup.6 up to 10.sup.9.

Abstract: A polyester film characterized by having many uneven units on its surface, each of said uneven units being composed of a fine projection and a depression existing therearound, and a process for producing aforesaid polyester film, which comprises pre-heating an unstretched polyester film containing internal particles attributed to an inert additive having an average particle diameter of 0.05 to 5 .mu.m and/or a catalyst residue to a temperature of 80.degree. to 120.degree. C.; stretching the film in a first axial direction to 2.6 to 3.5 times at a temperature of 110.degree. to 145.degree. C.; thereafter, stretching the film in a second axial direction at right angles to the first axial direction to 3.0 to 4.0 times at a temperature of 100.degree. to 150.degree. C.; and then heat-setting the stretched film at a temperature of 160.degree. to 240.degree. C.

Abstract: A biaxially oriented polyester film composed of a polyester composition comprising (A) 100 parts by weight of a substantially linear polyalkylene terephthalate having ethylene terephthalate as a main structural component, (B) 0.01 to 5 parts by weight of an ester of an aliphatic polyol having at least 4 hydroxyl groups in the molecule with an aliphatic monocarboxylic acid having at least 8 carbon atoms, and (C) not more than 0.8 part by weight of fine inorganic particles having an average particle diameter of not more than about 1 micron and being substantially insoluble in the polyalkylene terephthalate. The aforesaid biaxially oriented polyester film is useful as a base of magnetic tapes.