Prior to the fifteenth episode of the second season, The Last Mabelcorn, Billford was an obscure rarepair formed on the foundation that Ford and Bill definitely had a history of some sorts in the past.
The episode delved deeper into the past relationship of Ford and Bill, revealing they had once been friends and partners. Ford and Bill worked together on the portal (alongside McGucket who was left in the dark regarding Bill's Presence) and Bill was allowed to move in and out of Ford's mind as he pleased. Ford clearly trusted Bill, considered him a friend and idolized him, creating an entire shrine to him in his study and modeling parts (such as the windows) of what is now the Mystery Shack after Bill.
Because of their past friendship and Ford's obsession with Bill, they have been interpreted to be closer than the show let on or didn't outright say. The notion that Bill and Ford were in a romantic relationship at some point was not outlandish and had plenty of ambiguous canon evidence, and thus the Billford community grew and converted many shippers overnight.
Billford is canon, plenty of interactions between the two can be interpreted as romantic or flirtatious.Here are the reasons why it can be interpreted as being canon.
in the Journal #3 there are several pages describing Bill and his relationship with the author. Especially in some a page where Ford and Fiddleford go on a field trip; Stanford wrote that he saw Bill in the stars and that how the constellation (William) is his favorite.
Bill appears in Ford's dream and messes with him both physically and emotionally; some interpret this as flirtatious.
"Just let me into your mind, Stanford!" "Please, call me... a friend." Bill and Ford shake hands.
Ford's study resembles a worship shrine to Bill, emphasizing his obsession with Bill.
"Hiya smart guy! [...] And your name's Stanford Pines, the man who changed the world! But I'm getting ahead of ourselves! Let's relax! Care for a game of inter-dimensional chess? Have a cup of tea."
Bill refers to Ford with nicknames such as 'Sixer', 'Six Fingers', 'IQ', and 'Fordsy'.
Bill calls Ford cute when Ford insinuates he can stop him.
"I've been waiting an ETERNITY to have a chat face-to-face. Everyone, this Armageddon wouldn't be possible without help from our friend here. Give him a six-fingered hand! [...] Aw, don't look so sour, Fordsy, it's not too late to join me! With that extra finger, you'd fit right in with my freaks!"
Bill solidifies Ford into gold and uses him as a back scratcher.
Bill turns every single person in Gravity Falls into stone and uses them to build a throne, except for Ford, who he specifically keeps at his side at all times.
In Ford's words; Bill "seduced" him into thinking he was his friend.
Ford has six fingers, Bill has four; together they make ten.
In the episode commentary for “Land Before Swine” Stanley mentions that Ford is great at piano. This would mean that canonically, Bill and Ford share yet another similarity and are both excellent pianists.
Yet another parallel; when in dire, life-threatening situations, Bill and Ford both desperately cried out to Stan. “STANLEYYY!” - Both older Pines twins parallel Bill in different ways.
In the Book Of Bill (the new Gravity Falls book that came out not so long ago) Ford called himself a «Cipherholic».
In TBOB, Bill asks for a 'Sixer' at a place that sold drinks. When they didn't give him one, he cried and caused chaos.
In TBOB, there are many hints of them flirting with each other. There is even a page of them drinking together and Ford wrote 'one thing led to another'.
Ford is shown to have missed Bill after the betrayal.
In season one, Stan makes a joke. 'My ex wife still misses me, but her aim is getting better.' Ford says he misses Bill in TBOB, but 'his aim is getting better.'
While causing Ford pain, Bill says that Ford is his.
Mabel refers to Bill as a 'super needy ex'.
There are many hints in TBOB that heavily imply them being exes.
Bill gives love advice to the reader in TBOB. Stating: "if your date is still somehow unimpressed by your creaming and smells, the simply put them in a massive stone pyramid and sing and increasingly shrill song until they're mentally broken enough to confess their true feelings! FEAR and LOVE are right next to each other in the brain." This same scenario happened in Weirdmaggedon.
In TBOB Bill gives various advice on how to conquer the object of your affections, most of which is things he has done to Ford before, like giving rats to someone, trapping them in a tower, convincing them they need you, ect.
Ford gets a tattoo at Bill's request, thinking it means "wisdom" in his language. In fact, it literally translates to "If lost return to Bill".
On one of the diary pages you can see little Ford playing with a small yellow triangle with a spellbound look.
We learn in TBOB that when Bill hints at torture in the pyramid after taking Ford prisoner, instead of tormenting him, he gives Ford three rest days, even though he could have immediately started interrogating Ford. The electric shock was Ford's only injury. Therefore, it can be assumed that Bill has a soft spot for Ford.
In his book, in the section with the heading “my exes,” Bill has crossed out text. Ford is the most likely candidate for this role, given all the hints and facts.
Throughout his book, Bill recalls making extremely offended accusations against him. Also, almost a quarter of Bill's entire book is dedicated to Ford in one way or another.
There's a whole lot MORE than just that about Billford in this book, so we'll just wait until the electronic version come out. Stay tuned, shippers!
When inputting the code ‘divorce’ or ‘breakup’ into the new website, the ‘O’Sadleys’ logo comes up, which is where Bill went after falling out with Ford.