For those arguing that guidelines aren't rules, the DMG has these things to say about guidelines.
DMG page 5: "Chapter 5 presents guidelines and advice for running adventures set in dungeons, the wilderness, and other locales, and chapter 6 covers the time between adventures. Chapter 7 is all about treasure, magic items, and special rewards that help keep the players invested in your campaign." Chapters 5, 6 and 7 are all guidelines it seems.
DMG page 26: "Guidelines for creating a new background are provided in chapter 9, "Dungeon Master's Workshop." Making all those rules only guidelines.
DMG page 83(referring to encounter creation): "The preceding guidelines assume that you have a party consisting of three to five adventurers." So the primary thing the game is balanced around, the adventuring day and the encounter building "rules" are...........................guidelines! No rules there.
DMG page 87: "A group of monsters whose adjusted XP value constitutes an easy, medium, or hard challenge for the party, as determined using the encounter-building guidelines earlier in this chapter." More proof that all those encounter building rules are just guidelines.
DMG page 89: "For guidelines on generating monster-like stat blocks for an NPC, see chapter 9, "Dungeon Master's Workshop." So all the NPC creation rules are also just guidelines.
DMG page 103: "Chapter 8 provides guidelines for setting DCs and assigning statistics to doors and other objects." So all those rules on what stats objects have are just guidelines.
DMG page 105(and a lot of other pages): "Use the guidelines in chapter 8 to set an appropriate DC for any check made to spot or recognize a hazard." Isn't it interesting how chapter 8 and 9 are getting referred to as guidelines? I mean, those the chapters in the
Master of Rules section. It's almost as if the game is saying that guidelines and rules are interchangeable.
DMG page 122: "A successful DC 20 Strength check is necessary to pry the cover open. The cover can also be smashed open (determine the cover's statistics using the guidelines in chapter 8)." Again we have the hard statistics provided in the Master of Rules section being referred to as guidelines.
DMG page 129: "At your discretion, you can allow players to design their own magic items, using the guidelines in chapter 9, "Dungeon Master's Workshop."" So now we have the magic item creation rules being referred to as guidelines.
DMG page 236: "Or you can fill out the group with NPC followers, using the guidelines in chapter 4, "Creating Nonplayer Characters."" The chapter 4 NPC creation rules are now guidelines.
DMG page 241: "It's best to let players award their inspiration as they see fit, but feel free to
talk to them about following certain guidelines, particularly if you're trying to reinforce conventions of a certain genre." Guidelines are something to follow in order to reinforce things. Sounds like a rule.
DMG page 241: "A player follows whatever guidelines the group has agreed on for awarding inspiration." Again, guidelines as something to follow. A rule.
DMG page 260: "Chapter 3, "Creating Adventures," provides guidelines for designing combat encounters using experience points." So again we have all the rules for encounters and experience points being guidelines.
DMG page 263: "AS THE DUNGEON MASTER, YOU AREN'T LIMITED by the
rules in the Player's Handbook, the guidelines in this book, or the selection of monsters in the Monster Manual." Uh, oh! Now the PHB only has rules and the DMG only has guidelines.
DMG page 263: "This chapter contains optional rules that you can use to customize your campaign, as well as guidelines on creating your own material, such as monsters and magic items." Whew! What a relief. At least the optional rules in the DMG are rules.