Precision measurements have shown that the x-unit scale of x-ray wavelengths based on Mo Kα1=707.831 xu is about 20 ppm smaller than that in general use for Cu Kα1 and longer wavelengths, and for substantially all crystal parameters. Furthermore, the conversion from the Cu Kα1=1.537400 kxu to Å has been increased from 1.00202 to 1.002056+/-5 ppm. New techniques permit wavelength measurements to be made with relative probable errors of +/-1 ppm. New wavelength and crystal-parameter tables are now necessary, and this plus the newly developed precision technique provides the opportunity for introducing an x-ray wavelength standard. In an investigation to select the best wavelength to be designated as the standard, the following studies have been carried out: relative measurements of five wavelengths with five crystals, employing a least-squares adjustment; γ-ray study of single-crystal diffraction patterns and the use of a γ-ray wavelength as a standard; crystal imperfections and their effect on wavelength measurements; effect of chemical impurities on grating constants; and redetermination of the absolute wavelength scale. The requirements of a standard are analyzed and the advantages of the W Kα1 line listed. Its wavelength is 0.209010 0 Å+/-5 ppm on the absolute scale, and without error as the designated standard. With this standard, new wavelength tables (separate publication) have been prepared, which include a large number of new measurements of secondary wavelength standards used as reference lines in many original measurements. Probable errors on both the relative and the absolute scale have been included.