Dimensional Box

(This is a box.)
(You can put an item inside or take an item out.)
(The same box will appear later, so don't worry about coming back.)
(Sincerely, a box lover.)

A sign introducing the Dimensional Box to the protagonist in Snowdin

A Dimensional Box (JP: いじげんボックス) can be used to store 10 additional items, as the protagonist can only carry up to 8 items at a time in their inventory. There are a total of 9 physical boxes scattered around the underground, with 6 being available on any route, while 3 being exclusive to the Genocide Route. Alphys also upgrades the protagonist's Cell Phone, giving them access to another, virtual, Dimensional Box.

Function

Each Dimensional Box can hold 10 items. Items cannot be directly swapped between a box and the inventory, only deposited and withdrawn. If all boxes and the inventory are full, and the player wants to exchange an inventory item for a box item, they must consume, drop, or sell an item first to make the switch possible. If the protagonist has no items in the inventory or in the box, they cannot use physical Dimensional Boxes, which instead display unique dialogue, while using Dimensional Boxes from the Cell Phone displays empty items lists.

There are two Dimensional Boxes in Undertale, Dimensional Box A and Dimensional Box B. The protagonist can access physical Dimensional Boxes at various places in the underground, and each of them allows them to retrieve and deposit items into Dimensional Box A. Dimensional Box B is exclusively accessed on the protagonist's Cell Phone after Alphys upgrades it and does not have a physical counterpart. The first time the protagonist opens a Dimensional Box, they can take a Tough Glove from it free of charge.

In Neutral and True Pacifist Route, Alphys upgrades the protagonist's Cell Phone when they reach her lab in Hotland, after which, the protagonist can use their phone to access both Dimensional Box A and Dimensional Box B. On the Genocide Route, Alphys does not appear in her lab to upgrade the protagonist's Cell Phone; to compensate this, three additional physical boxes appear throughout the underground after Alphys's lab.

If the protagonist tries to use the Dimensional Boxes in the artifact room, the message "The box is aclog with the hair of a dog." is displayed, implying that the Annoying Dog is to blame. The protagonist cannot use the boxes until they leave the room or deploy the dog. This is done to prevent the protagonist from obtaining the Legendary Artifact.

Locations

Image	Loation	Map link
	Snowdin: Box Road, near the eastern exit. Has a box lover's sign next to it.	Map link
	Snowdin Town: In front of QC's shop and Snowed Inn.	Map link
	Waterfall: On the left side of the falling rocks room, with a box hater's sign next to it.	Map link
	Waterfall: Next to the telescope prank Sans puts up.	Map link
	Waterfall: Outside of Gerson's shop.	Map link
	Waterfall: Temmie Village, outside of the Tem Shop.	Map link
	Hotland (Genocide Route): MTT Resort, at the bottom-right corner of the lobby.	Map link
	New Home (Genocide Route): Asgore's Home, in front of the rightmost window.	Map link
	New Home (Genocide Route): Last Corridor, to the right of the SAVE Point.	Map link

Trivia

• A quote from the box lover is included in the trailer for the collector's edition release of Undertale for PlayStation 4 and PlayStation Vita.
• The game object for the boxes is named obj_sansbox, but their relation to Sans is unclear.
• The Dimensional Boxes are a reference to Brandish and Brandish 2, which features Dimensional Boxes A, B and C which exist as items in the player's inventory that allow the usage of another inventory's worth of space and allows moving and stacking of items.