A Gap (スキマ sukima) is a boundary controlled by Yukari Yakumo that relates to her ability to manipulate boundaries. She is able to create a cleft in thin air anywhere in Gensokyo and the Outside World to show this gaps, but it's never shown of what's behind the gaps.
The appearance of this gaps comes in many, with one having gashes in space, filled with a colour of purple, red and/or black. The cleft may feature red ribbons similar to the ones on Yukari's hair tied at both ends of the cleft. There maybe many eyes (and sometimes arms) that can peek out of the gaps. She is able to use the gaps as portals between two places, of which in Strange and Bright Nature Deity clearly shows the outside world through a gap, without the ribbons and in the shape of a mist.
The description of Yukari's Magic Eye "Laplace's Demon" spell card implies that Yukari can see using the eyes in her gaps, as well. Many of Yukari's portraits depict her holding, sitting on, or emerging from such gaps. Silent Sinner in Blue shows that when passing through a gap, it's as though you but your hand through non-transparent liquid. However, this gap was just above water, so it's not certain.
There are limits to the kinds of places Yukari can connect with her gaps; she required multiple trips to reach Heaven in Scarlet Weather Rhapsody, while in Silent Sinner in Blue she could only create a gap to the Moon under highly specific circumstances (and had to travel much of the distance to the Lunar Capital on foot). She is, however, capable of travelling between Gensokyo and the Outside World. Yukari can create gaps through which only she can pass, but various sources show her opening gaps for others to use.
- Perfect Cherry Blossom
The gaps were first seen used by Yukari Yakumo as she was sitting on one of them. However apart from this, nothing else was shown.
- Immaterial and Missing Power & Scarlet Weather Rhapsody
Yukari would occasionally use gaps during her fights or appearing with her opponent in a pre-fight scenario. When put to the ground after a good hit, she'll use a gap to get back on her feet, which would indicate laziness. She occasionally use gaps in her spell cards.
- Subterranean Animism
A gap was used for Reimu Hakurei's aid in Subterranean Animism, from which she is able to teleport from the one side of the screen to the other, which makes it the only occurrence that a gap was used in gameplay by another character besides Yukari.
- Antinomy of Common Flowers
Much like Immaterial and Missing Power & Scarlet Weather Rhapsody, Yukari uses gaps for entrances into battles, winning poses, and during the battle with certain attacks.
- Maribel Hearn appears to be able to use gaps in her own demand, although she can only see through them. Relation between her and Yukari (See Maribel's article) can support that it's the same use of gaps.
- In Highly Responsive to Prayers, there are portals that allow the Yin-Yang Orb to appear on another part of the screen. These act very similar to the gaps.
- Seija Kijin uses the tool in Impossible Spell Card called the "Gap Folding Umbrella" that works in a similar manner to Reimu Hakurei's usage of gaps in Subterranean Animism.
The Gaps in fandom are just like how Yukari treats it canonically, with using it as a portal and for sitting on, etc., except the usage of this is more advantageous. Yukari may use gaps to shoot danmaku at her opponent or use them as an "escape route" from a particular fight. She'll also use them to spy on other residents for no particular reason, and mocking around with various characters. She may also summon various objects from the Outside World such as a train or a prohibition sign that'll poke out of the gaps. The train is a popular meme, where Yukari is sometimes seen riding on top of a train from the outside world in fan-made videos, likely due to her most powerful spell card in Scarlet Weather Rhapsody and Touhou Hisoutensoku, in which she summons a train to run over her opponent.
- Alternative world
Although it isn't explicitly stated in canon, the Gaps are highly treated as an alternative dimension or another world of some kind with an infinite amount of space, similar to how Senkai is treated canonically. Common nicknames referring to this are "Yukari's Gap" (紫スキマ Yukari Sukima), "the Boundary" (境界 Kyoukai) and "Yukari Space" by westerners. Super Marisa World is a good example, as the world may also show to have normal Earth-like material and have the eyes looking directly at the player. On some cases along with Touhou Mother, various creatures are also shown.
On some games, gaps may be left across Gensokyo in order for the player to enter this world. It may be treated as a "route" for the player to cross and get to another part of Gensokyo. It may also show Yukari sucking a human from the outside world or other characters into this gloomy space.
- Behind the gaps
It's a real mystery as to what the appearance is on the other side of gaps, meaning if you walk around the gaps to look at the behind of it, what do you see? Due to the lining on the illustration in Perfect Cherry Blossom, fans may believe that the back is show as a wall of some kind, which is rarely shown in artwork. There's other theories including that you can do a whole 360 turn on a gap and you can still see the entrance through the gap, meaning it's regardless to what angle you enter it from.
- Gaps are useful for when certain fanworks would want to create a cross-over between the Touhou Project and another different work, in which Yukari has the capability to go to different fictional and non-fictional universes.
- Yukari's use to create gaps is interpreted being like the portal gun from the game Portal, of which many illustrations may show bizarre techniques used within portals. This lead to many different jokes about "mis-using" the ability in clever ways and/or mischievous pranks for her own selfish means.
- Fans tend to draw Yukari and her gaps with Bruno Buccellati from JoJo's Bizarre Adventure, who is able to open gaps (zippers) in the same fashion as Yukari.
- In some questionable NSFW works, she may use gaps to spy on others when they want a bit of "privacy".