Twenty-five years ago today, on September 13, 1990, a new show called Law & Order made its television debut. It premiered to strong reviews, and in addition to the show's unique cops-and-lawyers structure, ripped-from-the-headlines bombast, and tight pacing, it had one incredible signature "dum dum" sound (heard in the video above).

The sound notably appears over the show's title cards (like "The Western Docks, Monday, September 13"), and it's since left the Law & Order universe to become everything from a movie punchline to a T-shirt.

But it's not just a sound effect. Whatever you call it, the "doink doink," "dum dum," or "chung chung" is a piece of music that its creator never expected to become legendary.

The surprising secret ingredients of the Law & Order sound, including feet

The Law & Order sound, appropriately, still has a bit of mystery to it.

Mike Post, who created the sound, told the Archive of American Television in 2005 that the effect is a mashup of many different noises. "I sampled a jail door slamming, a couple other things — this clunk clunk, ching ching, chong chong thing, whatever you think it is."

He gave a little more detail to Entertainment Weekly in 1993: Most notably, the sound includes a sample of 500 Japanese men stamping their feet on a wooden floor as part of a large dance class.

The debate over the Law & Order sound's name

The name of the sound is also a source of debate. In that early EW article, Post calls it "the Clang," so that might have to be a contender. Fans seem to have coalesced around "chung chung" as the best onomatopoeic translation, and Post endorsed it as a possibility in at least one interview. However, "doink doink," "chong chong," and even "dum dum," have jockeyed for approval.

How the Law & Order sound happened

Long before he worked on Law & Order, Mike Post was a legendary television composer behind themes for shows like The A-Team, Doogie Howser, MD, and many others (you can listen to a compilation of his greatest hits).

In the late '80s, a former colleague, producer Dick Wolf, asked to meet with Post to pitch him his idea for a new type of TV crime show. From the beginning, Wolf had a unique vision for Law & Order's aesthetic (Post told the full story of the sound in the interview with the Archive of American Television). Wolf sought minimal music and a serious tone, so once he sent over the pilot, Post knew exactly how to put together the iconic theme song and score the show.

The classic sound effect was a footnote to the process. On a late call, Wolf gave Post a unique assignment: a sound for the location cards in the show. "Talk to sound effects," Post originally told Wolf. "Don't bother me with this." Wolf persisted, and Post eventually agreed to create something, even though it wasn't conventional music.

As a highly successful composer, Post never needed the sound effect to pay the bills. But it became so popular that Wolf insisted Post take him out to dinner.

And because the sound is technically a piece of music, it earns a royalty every time it's played. That's true on Law & Order, in its many spinoffs, or in other media cameos, like on fellow megahit The Big Bang Theory. That single sound effect has probably been worth a fortune.

That may be why Post prefers his own name for the iconic sound. "I call it the ching ching," he told the Archive of American Television, "because I'm making money off of it."