ENThe article addresses the contribution of Jonas Jablonskis, a contemporary of Jānis Endzelīns, to the standardization of Lithuanian orthography. The famous Latvian linguist was well acquainted with the standardization of Lithuanian orthography, saw similarities in the development of Lithuanian and Latvian orthography, and recognized Jablonskis’s contribution to the creation of a Standard Lithuanian. Jablonskis’s merits in the sphere of development of Standard Lithuanian are unquestionable, but the linguist’s input into the unification of orthography has not yet been thoroughly evaluated. It is debated whether Jablonskis contributed to the first collection of Lithuanian orthographic rules “Statrašos ramsčiai” (“Buttresses for Orthography”) (1890) by Vincas Kudirka. The journal “Varpas,” edited by Kudirka, Jablonskis (1893), initiated a discussion on Lithuanian orthography and proposed the letter ū for a long vowel. “Lietuviškos kalbos gramatika” (“The Grammar of the Lithuanian Language”) (1901) by Petras Kriaušaitis ( Jonas Jablonskis) is a seminal work in the history of the standardization of the Lithuanian language; the work also included the first printed version of the current Lithuanian alphabet in a complete form.Lithuanian orthography roughly established itself after the restoration of Lithuania’s independence in 1918, though two orthographic systems — Jablonskis’s orthography and Būga’s orthography — existed for a certain period (the orthography proposed by Kazimieras Būga was also supported by Endzelīns). Jablonskis actively participated in discussions on orthography, criticizing ill-founded proposals, but was also open to different opinions, highlighting the continuity of traditions and the necessity of unification. Jablonskis’s “Lietuvių kalbos gramatika” (“The Grammar of the Lithuanian Language”) (1922) made a substantial contribution to the establishment of orthographic norms. Though the linguist himself doubted some of his proposed norms, the orthography laid down in his grammar of 1922 established itself in public usage (official documents, periodicals, school). The major principles of Lithuanian orthography laid down in the above publication have been observed to this day. Keywords: alphabet, Lithuanian language, orthography, standardization of orthography, Jonas Jablonskis.