The design is based on the letterforms of capitalis monumentalis or Roman square capitals, as used for the inscription at the base of Trajan's Column from which the typeface takes its name. Trajan is an all-capitals typeface, as the Romans did not use lower-case letters. Twombly created the design taking inspiration from a full-size picture of a rubbing of the inscription. It is well known for appearing on many movie posters.
The capitals on the Column of Trajan have long been an inspiration to many artists and students of lettering. The calligrapher and type designer Edward Johnston in his book Writing & Illuminating & Lettering (1906) wrote that "the Roman capitals have held the supreme place among letters for readableness and beauty. They are the best forms for the grandest and most important inscriptions."
Twombly's translation of the Trajan inscription into type is quite crisp and faithful. Many looser interpretations (often with an invented lowercase) predate Twombly's, particularly Emil Rudolf Weiss' "Weiss" of 1926, Frederic Goudy's 1930 "Goudy Trajan," while Warren Chappell's "Trajanus" of 1939, while having similar forms for capitals has a markedly medieval lowercase. Many other examples of lettering and typefaces are based on Roman capitals, for instance lettering made under the influence of the Arts and Crafts movement in the nineteenth and twentieth centuries. Alastair Johnston's early review of Trajan noted this heritage, saying that it "outdoes anything old Fred Goudy ever produced."
Twombly retired from Adobe and type design in 1999, but Adobe has continued to release versions in consultation with her. The current OpenType release of Trajan is "Trajan Pro 3" and features a lower-case of small caps. Adobe has also released a "Trajan Sans" companion face, forming a font superfamily.
Trajan letterforms have been used for many years for signs in British public buildings, including government offices. Although other lettering is often used now, examples of Trajan signage can be seen at the entrances to the Houses of Parliament in London. Trajan also appears in the television series The West Wing. 
- Berry, John. "Trajan 3 Pro specimen" (PDF). Adobe Systems. Retrieved 18 October 2016.
- Riggs, Tamye (12 June 2014). "The Adobe Originals Silver Anniversary Story: Stone, Slimbach, and Twombly launch the first Originals".
- Zhukov, Maxim. "The Trajan Letter in Russia and America". Typejournal.ru. Retrieved 4 March 2017.
- Nash, John. "In Defence of the Roman Letter" (PDF). Journal of the Edward Johnston Foundation. Retrieved 13 October 2016.
- Tam, Keith (2002). Calligraphic tendencies in the development of sanserif types in the twentieth century (PDF). Reading: University of Reading (MA thesis).
- Johnston, Edward (1906). Writing & Illuminating & Lettering. Macmillan. pp. 268–269, 384, 391.
- Riggs, Tamye. "The Adobe Originals Silver Anniversary Story: A community perspective on the Originals program". Typekit. Adobe Systems. Retrieved 4 March 2017.
- Brown, Tim (15 November 2012). "Trajan Pro replaced by new, improved Trajan Pro 3".
- Brown, Tim (21 March 2012). "Inscriptional faces from Adobe Type".
- Typowiki: Trajan
- Trajan's Column, including detail of the capitalis monumentalis inscription.
- Etched in Stone, a humorous animation regarding the use (or overuse) of Trajan in film titles.
- Trajan is the movie font, a satirical video poking fun at the omnipresence of Trajan in the movie world.
- Adobe-Fonts: Trajan
- CastleType Fonts: Goudy Trajan
- Fonts in Use