'Law & Order' is an American police procedural and legal drama television series, created by Dick Wolf and part of the 'Law & Order' franchise. It originally aired on NBC and, in syndication, on various cable networks. 'Law & Order' premiered on September 13, 1990, and completed its 20th and final season on May 24, 2010. At the time of its cancellation, 'Law & Order' was the longest-running crime drama on American prime-time television. Its record of 20 seasons is a tie with 'Gunsmoke' (1955–75) for the longest-running live-action scripted American prime-time series with ongoing characters. Although it has fewer episodes than 'Gunsmoke', 'Law & Order' ranks as the longest-running hour-long prime-time TV series. 'Gunsmoke', for its first six seasons, was originally a half-hour program.
'Law & Order' spawned several spin-offs:
'Law & Order: Criminal Intent'
'Law & Order: Special Victims Unit'
'Law & Order: Trial By Jury'
'Law & Order: LA'
"Exiled: A Law & Order Movie"
It also crossed over several times with 'Homicide: Life On The Street', with one of that shows characters joining the cast of the spin-off 'Law & Order: SVU' (Detective John Munch). There have been several international adaptations, with 'Law & Order: UK' based on the original series and 'Law & Order: SVU' being interpreted by casts in Russia and France. (None of these three shows can be directly linked to the American franchise, but Munch got a mention in an episode of 'Luther' - one of the many reasons Detective Munch is the King of the Crossovers.)
So according to Wikipedia, 'Law & Order' began in 1990. And that's true - from the perspective of the Trueniverse. But the magic of the Toobworld Dynamic is that as soon as a show is broadcast, then its characters, locations, and the things which it has contributed to that alternate reality automatically have histories which can stretch back along the Toobworld timeline, sometimes even to antiquity.
For the fanficcers among Team Toobworld, you wouldn't have to just consider adventures and crossover for after their shows ended; you could fill in the blanks of their lives from before they debuted on the Toob. Prequels baby!
Think of the possibilities: Detective Lenny Briscoe may have gone to high school with Jean Davis ('Columbo' - "Requiem For A Falling Star"), Frank Barone ('Everybody Loves Raymond'), Elliot Carlin ('The Bob Newhart Show'), and the twin sisters Helen and Judy, better known by the married last names of Rosenthal and Geller ('St. Elsewhere' and 'Friends' respectively).
All of those options were chosen for the actors being born around the same time as Jerry Orbach.......
And as I mentioned, it wouldn't have to be just the characters. Their invented locations could also be used to link shows together earlier than expected. For 'Law & Order', that location would be the 27th Precinct.
So the series may have begun in September of 1990, but the Old 2-7 actually dates back farther than 1990. We saw evidence of its existence 20 years earlier!
In order to rush to the rescue during this case, Marshal Sam McCloud commandeered a squad car to race across town. And as you can see in this picture, it was assigned to the 27th Precinct.
A lot of possibilites in that: Detective Andy Sipowicz of the 15th (as seen in 'NYPD Blue') may have started out as a patrolman working out of the 27th. After the series 'NYPD' concluded, maybe Lt. Mike Haines was transferred to the 27th to be its new supervising officer. Paul La Guardia of the 14th Precinct could have been stationed there from even farther back, long before he worked with Detectives 'Cagney & Lacey'. And speaking of those Lady Blues, maybe Officer Casey Jones did some of her 'Decoy' work while stationed there. It could have been because of some big case solved by Detective Frank McNeil of the 27th which got him the promotion to Captain, stationed at the 11th (with his best detective being Theo 'Kojak'.)
As a detective on the other coast would often say: "Just one more thing....."
O'BSERVATION: That episode title of "Murder Arena" was not in the original list from the series. It's a two-hour combination of two other episodes ("The Concrete Corral" and "Walk In The Dark".) I consider it as just another perspective of the events in McCloud's life rather than having to ship it off to another TV dimension. There was no similarity in the cases, but it does show that just like in real life, TV characters have to juggle the events of their lives; not everything happens in an orderly fashion.