This depends on what you mean by "early 20th century", but the short answer to your question is that movie trailers as we generally understand them -- compilations of scenes from the film used for advertisement -- didn't come about until the late-1910s.

Every casual history on the subject you can find will claim that the first trailer came about in 1913, though this is misleading. A manager for the Marcus Loew theater chain did create something broadly identifiable as a "trailer" by today's standards, however it was not an advertisement for a movie; rather, it was a one-minute ad for an upcoming Broadway musical, which included rehearsal footage from the show. Close enough, I suppose, but certainly not a "movie trailer".

1913 also saw another precursor to movie trailers, however. In that year, Selig Polyscope produced one of the first serials, The Adventures of Kathryn, a 13-chapter series of 1-reels films (roughly 10 minutes). As would become standard in serials, each chapter would end on a cliffhanger, and in order to entice audiences to come back for the next installment, Selig included a preview of the next chapter.

Keep in mind, feature-length films are only beginning to arise at this time; 1- and 2-reel films (under 20 minutes) are still the standard, so serials were one of the few instances where you'd have much incentive to advertise a short. But as feature-length films rose in popularity over the next few years, the studios of the time did begin experimenting with advertisements for upcoming films. Initially, however, they were not found to be very effective. And in hindsight, the reason is obvious.

Like with Selig Polyscope's Adventures of Kathryn, these advertisements were shown after the film, as an enticement to come out to future movies, hence, the name "trailers". But just as today, audiences would get up and leave as soon as the film was over, which meant these ads weren't reaching many viewers. And since they were not very effective, studios quickly questioned whether they were worth the time and money to produce.

But while the studios were lukewarm on trailers, entrepreneur Herman Robbins recognized an opportunity, as theaters were still interested in advertising materials. So Robbins began to use available promotional stills combined with text and intertitles to produce short advertising clips, and he sold them directly to the theaters themselves. This was the beginning of the National Screen Service, the company that would dominate movie trailers (as well as other forms of movie marketing) for the next 40 years.

This was initially done without any permission from the studios themselves, however since the studios were happy to outsource the production of these promotional materials, The National Screen Service soon began to sign contracts with each of the major Hollywood studios. Working directly for the studios, the NSS now had access to footage from upcoming films, and they soon standardized the movie trailer format that would remain common up through the 1950s. Standard trailer conventions would include screen wipes, bold text that would fly onto the screen, and later, with the coming of sound, an enthusiastic voiceover.

But the trailers were still "trailing" the film throughout the 1920s, as it wasn't until the 1930s that they would get moved to the front. As the Great Depression took hold, theaters began offering more than just a feature film, instead providing a whole evening's worth of entertainment: a newsreel, a cartoon short, maybe a serial installment, and eventually two feature films (the famed "double feature"). With all the pre-feature material, it became easier to couch a couple advertisements for upcoming films among the shorts and newsreels, and the modern idea of the trailer finally comes together.

My quick googling couldn't find any original trailers from the 1920s, but to give you a idea of what they looked like, here's a trailer from 1931's Frankenstein and here's one for 1935's Top Hat.

For more reading, Lisa Kernan's Coming Attractions: Reading America's Movie Trailers (2004, University of Texas Press) is pretty good. She doesn't lay out much of a historical timeline, but she does delve into the conventions and formats of different eras, including a chapter on the Classical Hollywood era. Otherwise, you kinda have to scour books on broader film history.