Multifilament yarn having novel configuration and a method for producing the same

A polyester yarn composed of a plurality of individual fibrous materials, each of said fibrous materials provided with thicker cross-sectional portions, thinner cross-sectional portions and intermediate thickness-size portions randomly distributed along the axial direction thereof, said by the following four conditions,

• a. a distribution curve of the cross-sectional area of said individual fibrous materials is deviated to the thinner side,
• b. the degree of variability, V/S, of said cross-sectional area of said individual fibrous materials, where V is the standard deviation and S is the mean value, is in a range between 7% and 30%,
• c. in the distribution of the cross-sectional areas of said fibrous materials, if the range of distribution is divided in such a way that the distribution range in the thicker side from the average value is divided by a width corresponding to 1/2 of the standard deviation thereof, the distribution frequency in any class defined by the above-mentioned method of division is less than three times the distribution frequency in a class adjacent to said specific class in the thinner side of the distribution.
• d. the standard deviation of the average cross-sectional area of individual fibrous materials in optional cross sections of said multifilament yarn is smaller than the quotient of the standard deviation of said fibrous materials divided by the one-fourth power of the average number of said fibrous materials constituting said optional cross sections of said multifilament yarn.