Mewni is an alternate dimension, the birthplace of Star Butterfly and the native dimension/realm of the entire Butterfly family. It is a medieval land divided into several kingdoms ruled by kings and queens of various species. The most notable kingdom is Butterfly Castle, formerly ruled by King and Queen Butterfly. After the events of Conquer, Eclipsa Butterfly once again rules as Queen of Mewni with her daughter, Meteora, as the heir.
Mewni looks like a typical fairy tale locale, with various magical creatures and a royal family. Families of nobility include the Butterfly family, the Pony Head family, and the Johansen family. As seen in "Diaz Family Vacation", the occupants residing in the Butterfly Kingdom (ruled over by the Butterfly family) and their place of residence appears to depend on their status in the hierarchy: peasants occupy the lowest land (bottom level), having dirty streets and widespread poverty; then presumably middle-ranking residents occupy the middle section (second level), which is less atrocious than the previous section; and the nobles occupy the highest part of the land where the castle is prominent (top level). This hierarchy appears to have been somewhat dissolved over the course of Eclipsa Butterfly's second reign.
Beyond the Butterfly family's kingdom is a swampland, where Mewni's indigenous monster population were forced to flee after the first settlers forced them off of their land. Since Eclipsa reclaimed the throne, however, the monsters have begun reintegrating themselves back into Mewman society. Mewni's main crop is corn, leading to being heavily integrated into Mewni society.
The sky on Mewni is pink, in contrast to the blue sky on Earth; however, following the merging of Mewni with Earth, the sky is now a mixture of blue and pink.
According to Star and Marco's Guide to Mastering Every Dimension, the royal lineage of the Butterfly family is "matrilineal", meaning the queen holds more authority than the king, and the throne is handed down from the queen to her firstborn daughter. As a result, King River Butterfly (born River Johansen) took his wife's last name upon their marriage. In one case, Prince Jushtin Butterfly was briefly in line to succeed his mother Skywynne as Mewni's ruler, but the birth of his sister Solaria caused him to be superseded.
- Butterfly Kingdom: Capital of Mewni, home to the Butterfly family. As of "Conquer", the capital lies in ruins.
- Butterfly Castle: The castle of the royal family.
- Tapestry Room: A room in where the previous queens of Mewni are depicted in tapestry. Star calls it the "Grandma Room".
- Butterfly Groundlands: The area immediately surrounding Butterfly Castle, which includes a village.
- Marketplace: A common marketplace found within the village.
- Butterfly Castle: The castle of the royal family.
- Castles: A pair of castles near to Butterfly Castle.
- Castle Avarius: The castle in which Ludo and his army previously dwelled. Used to belong to his family before he changed the locks while they were vacationing and took it for his own. It was destroyed in "Storm the Castle", and later rebuilt by Dennis Avarius under his ownership in "Princess Quasar Caterpillar and the Magic Bell".
- Forest of Certain Death: A dangerous forest occupied by carnivorous plants and monsters, seen in "Diaz Family Vacation".
- Rat bar: A small bar for rats to socially interact and eat porridge.
- Buff Frog's house: The house where Buff Frog lives with his adopted tadpole children.
- Corn mine: An underground mine that Ludo used to make monsters grind corn for him. After Ludo abandons it, a group of "alternative monsters" moves in sometime before "Raid the Cave".
- Monster Temple: Ludo's new home in the mountains that he and his rats, eagle, and spider moved into sometime after "Is Mystery". Destroyed in "Starcrushed". In "Monster Bash", it is revealed to be where Eclipsa Butterfly and her monster husband lived and raised their daughter, Meteora. In season 4, Eclipsa has the temple rebuilt, refurbished, and made into her new castle.
- New Monstertown: A growing village established at the base of the Monster Temple.
- Sanctuary: A magic sanctuary/hiding place for the Butterfly family and Magic High Commission.
- Johansen Kingdom: The kingdom of the Johansens, King Butterfly's born family. Located to the east of Butterfly Castle.
- Pigeon Kingdom: The kingdom of the pigeons. Located to the west of Butterfly Castle.
- Jaggy Mountains: The kingdom of the Jaggy royal family.
- Forest of Unlikely Spider Bites: The kingdom of the Spiderbite royal family.
- Underworld: A sub-level kingdom located deep in the Mewni underground. Ruled by the Lucitors, Tom's family.
- Cloud Kingdom of the Pony Heads: A kingdom that spans Mewni's entire sky, just below outer space. Ruled by the Pony Heads, Pony Head's family.
- Kingdom of the Waterfolk: An undersea kingdom that encompasses all of Mewni's oceans. Ruled by the Waterfolk.
- Lake of Endless Suffering
- Dam of Constant Sorrow
- Bridge of the Inevitable Hereafter
- Village of Monsters: A small village in Mewni where a community of monsters resides.
- Pie Island: The home of the Pie Folk. It contains a village, a port, and a volcano (filled with lukewarm tomato sauce) where the Pie Folk make their pies.
- Village of Mewmans: A Mewman settlement, comprising of Moon and River Butterfly, and of Mewmans who were either forced to return their land to the monsters their ancestors took said land from, or simply left out of disgust over Eclipsa's rule.
- Brittney Wong demeaningly refers to Mewni as "Moo-ni" in "School Spirit". Angie Diaz also mistakenly pronounces it this way in "Sophomore Slump" despite previously saying it correctly in "Diaz Family Vacation".
- In the episode "Match Maker", it is revealed that kids in Mewni do not take tests.
- Daron Nefcy expands on this further on her Twitter, stating that there are no classrooms on Mewni.
- Contrary to this, however, Jushtin Butterfly had a math tutor, according to Star vs. the Forces of Evil: The Magic Book of Spells.
- Skywynne is also mentioned in The Magic Book of Spells to have opened an institute of some sort on Mewni.
- In the episode "Lobster Claws", Lobster Claws reveals that he and his fellow monsters, and Ludo, also come from Mewni.
- Inhabitants of Mewni are called "Mewmans", a portmanteau of "Mewni" and "humans", as stated in "Mewnipendance Day".
- "Sleep Spells" shows that the Mewnian idea of "Psychology" is launching someone into the distance with a catapult.
- "Fortune Cookies" reveals that one of the foods of Mewni are talking calzones that tell the eater how they are going to die.
- In "Star on Wheels", Star mentions that there are invisible goats on Mewni who pull various vehicles.
- "On the Job" reveals that Mewni's monarchy is segregationist. While the Mewmans know how to grow corn, they refuse to openly share it with non-Mewman residents.
- Though "Diaz Family Vacation" shows widespread poverty amongst the lower levels, later episodes do not show as much grime or filth amongst the citizens.
- In "By the Book", when Glossaryck instructs Star to point her arms out at 11:00, Star asks "Earth 11:00 or Mewni 11:00?", and Glossaryck answers, "They're the same," suggesting a similar time notation between the two dimensions.
- However, in the #StarLIVE Live Chat (around 12:55), Star suggests that Mewni's time notation is different than Earth's. It is most likely that the voice actors in the live chat forgot about this and got it wrong as they were just improvising.
- As of the end of Age of Magic "Cleaved", Mewni has been ruled by 37 queens (not counting the repeated reigns of Moon and Eclipsa), 12 of them identified by name.
- Daron Nefcy (2016-08-22). Not in the same way as on earth. No classrooms on Mewni!. Twitter. Retrieved on 2017 November 19.
- #StarLIVE w/Star & Marco!!!. Youtube (2017-07-17). Retrieved on 2018 April 23.
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