Cramped Movement
PG. 190 PHB, Movement and Position
A largely overlooked Rule that actually comes RAW and from an official source. GM's that do not enforce this rule play with a much more casual game that isn't as tactical.
Difficult Terrain
Combat rarely takes place in bare rooms or on featureless plains. Boulder-strewn caverns, briar-choked forests, treacherous staircases---the setting of a typical fight contains difficult terrain.
Every foot of movement in difficult terrain costs 1 extra foot. This rule is true even if multiple things in a space count as difficult terrain.
Low furniture, rubble, undergrowth, steep stairs, snow, and shallow bogs are examples of difficult terrain. The space of another creature, whether hostile or not, also counts as difficult terrain.
Moving Around Other Creatures
Youc an move through a nonhostile creature's space. In contrast, you can move through a hostile creature's space only if the creature is at least two sizes larger or smaller than you. Remember that another creature's space is difficult terrain for you.
Whether a creature is a friend or an enemy, you can't willingly end your move in its space.
If you leave a hostile creature's reach during you rmove, you provoke an opportunity attack, as explained later in the chapter.
