In this work we present spectroscopic observations centered in the spectral lines most widely used as optical indicators of chromospheric activity (Hα, Hβ, Ca ii H & K, and He i D3) in a sample of F, G, K and M chromospherically inactive stars. The spectra have been obtained with the aim of providing a library of high and mid-resolution spectra to be used in the application of the spectral subtraction technique to obtain the active-chromosphere contribution to these lines in chromospherically active single and binary stars. This library can also be used for spectral classification purposes. A digital version with all the spectra is available via ftp and the World Wide Web (WWW) in both ASCII and FITS formats. Based on observations made with the Isaac Newton telescope and the William Herschel Telescope operated on the island of La Palma by the Royal Greenwich Observatory at the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofisica de Canarias, and with the 2.2 m telescope of the Centro Astronomico Hispano-Aleman of Calar Alto (Almeria, Spain) operated jointly by the Max Planck Institut fur Astronomie (Heidelberg) and the Spanish Comision Nacional de Astronomia. The spectra of the stars listed in Table protect ef{tab:par} are also available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html