<P> In November 2014, Berlanti expressed interest in Supergirl existing in the Arrowverse, the same universe as his other series Arrow and The Flash, and in January 2015, The CW president Mark Pedowitz revealed that he was also open to a crossover between the series and networks (due to Berlanti executive producing all three and The CW being co-owned by CBS). However, CBS Entertainment chair Nina Tassler stated that month that "those two shows are on a different network . So I think we'll keep Supergirl to ourselves for a while ." In August 2015, Tassler revealed that while there were no plans at the time to do crossover storylines, the three series would have crossover promotions . </P> <P> Pedowitz regretted passing on the series when presented it in mid-2014, saying, "We hadn't launched The Flash yet, we weren't ready to take on another DC property . In hindsight we probably should've gone that direction...Sometimes you lose great shows ." In January 2016, during the Television Critics Association press tour, he said that The CW was still interested in a crossover with Supergirl if the producers could find a way to do it, and Berlanti added that while no official conversations had taken place, internal ones had concerning how a crossover would work . He also noted that for a crossover to happen during Supergirl's first season, it would have to be figured out "in the next month or so". Glenn Geller, Tassler's successor at CBS, then stated on the matter, "I have to be really careful what I say here . Watch and wait and see what happens ." </P> <P> On February 3, 2016, it was announced that Grant Gustin, who appears as Barry Allen / Flash on The Flash, would appear in the eighteenth episode of the first season, "Worlds Finest". While no plot details on the episodes were released at the time, Ross A. Lincoln of Deadline Hollywood noted that "the in - universe reason" for the crossover was due to Barry's ability to travel to various dimensions, thus implying that Supergirl exists on an alternate Earth to the Arrowverse in a multiverse . The Flash episode "Welcome to Earth - 2" confirmed this, showing an image of Benoist as Supergirl during a sequence where characters travel through that multiverse . The earth that the series inhabits is Earth - 38 in the Arrowverse multiverse, and has been informally referred to as "Earth - CBS" by Marc Guggenheim, one of the creators of Arrow . </P> <P> During the second season, Supergirl appears in "Invasion!", a crossover episode of The Flash, Arrow and Legends of Tomorrow, when she's recruited by Barry Allen and Cisco Ramon at the end of "Medusa" to help fight off an invasion by the Dominators . Supergirl and The Flash also featured in a musical crossover, featuring several covers of existing songs along with two original numbers . Similar to "Invasion!", the crossover begins at the end of the Supergirl episode "Star - Crossed" and primarily takes place during The Flash episode "Duet", featuring the Music Meister as the antagonist who puts both The Flash and Supergirl in a shared hallucination . After "Invasion!", Guggenheim felt "If there's an appetite for it from the fans and from the network," the crossover next year could be "a proper four - part crossover ." </P>

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