Leibniz said that his goal was an alphabet of human thought, a universal symbolic language (characteristic) for science, mathematics, and metaphysics.
According to Couturat: > In May 1676, he once again identified the universal language with the characteristic and dreamed of a language that would also be a calculus—a sort of algebra of thought. > > (1901, chp 3.).
Louis Couturat. Louis Couturat was a French logician, mathematician, philosopher, and linguist. Couturat was a pioneer of the constructed language Ido. - wikimedia 
This characteristic was a universalisation of the various "real characteristics". Couturat wrote that Leibniz gave Egyptian and Chinese hieroglyphics and chemical signs as examples of ''real characteristics'' writing - wikipedia
In a footnote, Couturat added: > Elsewhere Leibniz even includes among the types of signs musical notes and astronomical signs (the signs of the zodiac and those of the planets, including the sun and the moon). It should be noted that Leibniz sometimes employs planetary signs in place of letters in his algebraic calculations > > Couturat, 1901, chpt. 3