U2's lyrical rules
U2 banned the word "baby" from their songs.
The 'One' rockers used to impose strict rules on their music, but after 20 years, "loosened up" and released an album called 'Achtung Baby' in 1991 and have subsequently allowed the use of previously-forbidden words and phrases.
Frontman Bono said: "Limitation leads to innovation and a rule book is useful even if you eventually tear it up.
"U2 had musical notes we wouldn't use (certain bendy ones from a blues scale on guitar) and lyrics we decided were against the law like 'Baby' and 'Crazy'.
"It took 20 years but we eventually loosened up on both and included the forbidden 'baby' in the title of our seventh album and had it appear 23 times in the music."
Looking back, Bono admits he wishes the group had had more rules, especially when it came to some ill-advised haircuts.
He added: "In truth, we needed more rules. Though shalt not wear a mullet for a start, a rock star should not look like his hair is ironed."
Though the 'Sweetest Thing' hitmaker admits he and his bandmates have overlooked some behaviours that could have got one another fired, they always had a pact that playing golf was a punishable offence - though he suspects it is a rule that has been broken.
He said: "When we started we committed to the principles of a golf-free band. Many mistakes were allowed for some of us who did terrible things that deserved firing, but nobody ever played golf. Except, we think, The Edge."
Bono's best piece of advice came from the group's "oracle", drummer Larry Mullen, Jr.
Writing for Q magazine, he explained: "Question everything. Like many things, I learned this from the band oracle, Larry Mullen Jr.
"He took me aside after a gig and told me he needed to tell me something extremely important. He took my hand. Looked deeply into my eyes and said, 'Bono, question everything.' I said, 'Why?' "