Playing on a Grid
If an encounter involves combat, it’s often a good idea to track the movement and relative position of the participants using some form of grid to display the terrain and tokens to represent the combatants. When a character moves on a grid, every square of the play area is 5 feet across in the game world. Hence, a creature moving in a straight line spends 5 feet of its movement for every map square traveled.
Because moving diagonally covers more ground, you count that movement differently. The first square of diagonal movement you make in a turn counts as 5 feet, but the second counts as 10 feet, and your count thereafter alternates between the two. For example, as you move across 4 squares diagonally, you would count 5 feet, then 10, then 5, and then 10, for a total of 30 feet. You track your total diagonal movement across all your movement during your turn, but reset your count at the end of your turn.
Entering a Square
To enter a square, you must have at least 1 square of movement left. If a square costs extra movement, as a square of difficult terrain does, you must have enough movement left to pay for entering it. For example, you must have at least 2 squares of movement left to enter a square of difficult terrain.
Corners
Diagonal movement can’t cross the corner of a wall, large tree, aor other terrain feature that fills its space.
Ranges
To determine the range on a grid between two things (whether creatures or objects) start counting squares from a square adjacent to one of them and stop counting in the space of the other one. Count by the shortest route.