Important mediæval manuscripts and later literary and historical manuscripts, [12 cent, first ½]-[late 19 cent.], reflecting the collecting interests of the Maurice and Owen families of Clenennau and Brogyntyn.
They include an important mid-fifteenth century miscellany of prose and verse in Middle English (Brogyntyn MS II.1); English and Welsh poetry; plays; astrology and prophecies; chronicles of the history of Britain, one of which is a thirteenth century version of Historia Regum Britanniæ of Geoffrey of Monmouth (Brogyntyn MS I.7); a lute book, [c. 1595] (Brogyntyn MS I.27); a copy of the laws of Hywel Dda [1625x1632] (Brogyntyn MS I.12); legal precedents and other papers of legal interest in Latin and English; pedigrees, genealogy and heraldry of North and South Wales families; religious treatises, prayers, devotions and sermons; a seventeenth-century Latin-Welsh dictionary and other manuscripts of linguistic interest; extracts from classical literature; commonplace books; academic notes; copies of significant historical letters and documents; political tracts; moralistic and philosophical works; memoranda, journals and private papers of members of the Anwyl and Owen families; a few Brogyntyn estate and trust papers, 1727-1792; and notes on public offices and official papers deriving from the administration of Oswestry Corporation, 1660, 1673. Some ancilliary materials, [19 cent, second ½]-[1960s], mostly correspondence and notes relating to individual manuscripts, are also included (MSS I.27a, II.1a, II.10a, II.22a, II.42a, II.54(h), II.56a).

Manuscript collections containing original and transcribed poetry and prose, 1543-[20th century]; religious works, 1611-1914; biographies, chronicles, archaeological and historical notes, works on travel, folklore, recipes, grammars, vocabularies and philological notes, 1673-1900; rent-rolls, parish registers, tithe account books, bidding books, minute books and registers of various societies, 1608-1863; pedigrees and genealogical notes, [17th century]-1871, commonplace books, [18th century]-1904, volumes of press-cuttings; manuscripts in the hand of J. H. Davies, including transcripts of and indexes to Welsh poetry, bibliographical notes, literary and legal extracts, pedigrees, biographical data and notes on place-names, 1891-1923; autograph letters from prominent Welsh literary figures, 1674-1921; autograph manuscripts of John Jones, Gellilyfdy, Dafydd Ellis of Criccieth, Eben Fardd (including diaries, 1836-1859), Peter Williams, D. Silvan Evans, Thomas Richards and Myrddin Fardd.

A volume containing the first part of a transcript (continued in Brogyntyn I.10) by John Edwards, Plas Newydd, Chirk, of part of Thesaurus Linguæ Latinæ et Cambrobritannicæ... by 'Syr' Thomas Wiliems, Trefriw (now NLW Peniarth MS 228).
The volume contains the title page, dated 1608, the list of authorities cited, 'Prif Gaerae Ynys Brydain gynt', 'Tri dyfal gyfangan', and the dictionary from A to D. The contents correspond to Peniarth MS 228, vol. i, except that Thomas Wiliems's preface is wanting, probably as a result of six leaves being excised at the beginning of the volume between ff. iv and v. The text is written in a neat italic hand, two columns on each page; the Latin words are written in red ink, the Welsh definitions in black, quotations from Welsh literature in green, and the names of authors and titles of the works quoted in purple. According to a note on f. iv John Edwards paid nine shillings for paper for his transcript in 1607; he probably commenced transcribing soon afterwards in 1608, which is the date written on Thomas Wiliems's title-page, although Wiliems completed his dictionary on 2 October 1607. The monograms 'IHS' and 'MRA' on ff. v and 1 and 'Emanuell IHS' on f. iv indicate that John Edwards, like Thomas Wiliems, was a Roman Catholic. Because of his refusal to take the Oath of Allegiance in 1613 two-thirds of Edwards's estate was confiscated, which resulted in his being involved in litigation until the end of his life, a situation which probably explains why he abandoned transcribing the dictionary.

A volume containing the second part of a transcript (begun in Brogyntyn MS I.9) by John Edwards, Plas Newydd, Chirk, of part of Thesaurus Linguæ Latinæ et Cambrobritannicæ... by 'Syr' Thomas Wiliems, Trefriw (now NLW Peniarth MS 228).
The contents continue the dictionary from E to Indeclinabilis and corresponds to Peniarth MS 228, vol. ii, ff. 2-113. The text is written in a neat italic hand, two columns on each page; the Latin words are written in red ink, the Welsh definitions in black, quotations from Welsh literature in green, and the names of authors and titles of the works quoted in purple. Spaces are sometimes left for the later addition of the Welsh quotations.