A grammar in five parts written about 1791 by Humphrey Thomas, schoolmaster, and his brother, David Thomas (Dafydd Ddu Eryri), poet (1760-1822). The preface is dated from Carnarvon, March 25th, 1791. Part I ends on p. 86, Part II on p. 168, Part III on p. 248, Part IV on p. 312 and Part V on p. 382. A Grammar by H. and D. Thomas is noted in Llyfryddiaeth y Cymry 1794, No. 10, and on p. 45 of Awdlau ar Destynau Cymdeithas y Gwyneddigion, London, 1791, by D. Thomas, there is a reference to the proposed publication of this MS, which bears the following title on p. 3 : Arweinydd ir Gymraeg a amcanwyd i dywys yr anhyddysg i lwybr gwybodaeth fuddiol, ac i gyfarwyddyd helaeth yn ardderchawgrwydd yr Iaith a chywreinrwydd ei Barddoniaeth . . . gan Humphrey Thomas, Athraw Ysgol, a Dafydd Thomas, Bardd.