Typography was his first interest, when Herbert Bayer came to the Bauhaus in 1921, influenced by De Stijl and Laszlo Moholy-Nagy.

universal by herbert bayer. It was created in 1925 at the Bauhaus in Dessau (this is a later version) and was a radically modern design: no serifs, as rational and functional as possible and lower-case letters only.
Bayer wanted to simplify the typeface and do away with ‘unnecessary’ distinctions. It is important to note that ‘Universal’ was originally more of an experimental typographic design than a fully developed lead typeface. Over a number of years, Bayer developed several revisions and variations on this idea. Although the design was never cast as a typeface, it became a model for Bauhaus students, colleagues and followers alike. Many were introduced to the design after it appeared in the Bauhaus publication Offset No. 7. Today’s digital versions are mostly later reconstructions or interpretations.
Bayer suggested to write with the “kleinschreibung” (lower case lettering) and to use the capital letters for posters and displays. “This would save space and money”, so Bayer said, because there was only one range of typeface. There was a four-line explanatory text on lower-case lettering at the bottom of the letterhead.

Soon the publications from the Bauhaus used only small letters minuscules. In 1925, Herbert Bayer took over as head of the Bauhaus print shop and subsequently revamped the visual identity of the Bauhaus in its printed materials. For the letterheads, which were now printed in DIN sizes, he adapted Moholy-Nagy’s designs and used a more uniform typography and lines as design elements.
wir schreiben alles klein, denn wir sparen damit zeit. außerdem: warum 2 alfabete, wenn eins dasselbe erreicht? warum großschreiben, wenn man nicht groß sprechen kann?
“Because it is inconsistent in language use to write differently from the way one speaks. We do not speak in capital letters, so we do not write them either. And: does one not say the same thing with one alphabet as with two? Why merge two alphabets of completely different character into a single word or sentence, thereby making the text look disharmonious? Either capital or lower-case. Capital letters are illegible in a sentence. Hence the use of lower-case letters. And: if we think of the typewriter, restricting ourselves to lower-case letters brings great convenience and saves time. And if we take this further, eliminating capital letters altogether would simplify matters and save time.” From: Bauhaus magazine 1.1926

Bayer himself adapted versions of the Universal Typeface for his own work, such as for the logo of the fashion magazine die neue linie. 1930–1936
By the way: Later, the National Socialists found the use of lower-case letters by the Bauhaus followers so irksome that they instructed the city authorities not to open letters written in lower-case, but to destroy them.