Open 4E

Core mechanic

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The Core MechanicEdit

As you play your character, you will describe things that you want to do. The Game Master’s job is to help determine what happens as a result of your actions. In this game, all of these situations are resolved through a single mechanic, called a check: you roll 1d20, add modifiers based on your competency or the situation, and announce the check result. Usually, the Game Master will compare this check result to a difficulty number based on the situation and the inherent difficulty of what you’re trying to accomplish. In many common situations, the process for determining the check modifier and difficulty have been worked out in detail, but always remember that you and the GM can decide to arbitrate situations that haven’t been specifically spelled out in the rules, or modify the check or difficulty of well-established rules based on special circumstances.


If your check result is equal to or greater than the difficulty, or if you roll a natural 20, you succeed. If you roll a natural 20 and your check result is greater than the difficulty, you achieve a critical success.


If your check result is less than the difficulty, or if you roll a natural 1, you fail. If you roll a natural 1 and your check result is less than the difficulty, you suffer a critical failure.


Some tasks are easier to perform than others. In general, the base difficulty of a task will be one of five numbers.

  • Trivial tasks have a Difficulty of 5. These are actions that, in general, the GM should just declare that you succeed at rather than having you roll at all. A GM might require a roll on a trivial task in extraordinary circumstances.
  • Easy tasks have a difficulty of 10. These are actions that an average, untrained person is expected to have an even chance of accomplishing successfully. Examples: walking eight hours along a paved road without rest, assembling a puzzle, applying bandages to a wound, remembering the name of the capital city of a neighboring province.
  • Moderate tasks have a difficulty of 15. These are actions that the average person can pull off with some luck, and which a competently trained person is expected to have an even chance of accomplishing. Examples: walking eight hours through a hilly woodland without rest, assembling a model ship, sewing up a wound, remembering the name of the capital city of every province in your country. If no task difficulty is specified, assume that the task has a moderate difficulty.
  • Hard tasks have a difficulty of 20. These actions require a good amount of expertise, training, and natural ability to pull off well, and are only accomplishable through extraordinary luck otherwise. Examples: carving a path through a densely overgrown jungle for eight hours without rest, assembling a clockwork pocketwatch, digging a barbed arrowhead out of a wound without causing more damage, remembering the name of every capital city of every country in the world.
  • Legendary tasks have a difficulty of 25. These are tasks that a normal person might accomplish once in a lifetime, and which are noteworthy even for heroes. Examples: assembling a working clockwork model of the solar system, performing open-heart surgery without the aid of magic or modern medicine, remembering the name of every city in your country with a population of 50 or more.
  • Nigh Impossible tasks have a difficulty of 30 or higher. These tasks seem utterly impossible to perform, but the truly gifted might pull them off once in a lifetime. Examples: assembling a thinking, feeling clockwork man, reviving the dead, remembering the name of every city the world and a specific historical fact about each of them.


Each time you make a check, one or more modifiers will most likely apply to the roll. Modifiers go in two directions - bonuses are added to your die roll, while penalties are subtracted from it.


Most bonuses come from a particular source; these modifiers will have an adjective to describe them, such as feat bonuses or class bonuses or circumstance bonuses or racial bonuses. Bonuses are always positive; if an effect would cause a bonus to be negative, treat it as zero instead.

If several bonuses apply to the roll you are about to make that all have the same adjective, only the highest one applies, and only once. For example, if you have a +2 class bonus to saving throws, and a +5 class bonus to saving throws vs. charm effects, then you apply the +5 modifier when making a saving throw vs. a charm effect, not the +7 that you would get from adding them together. However, if you have two different bonuses, such as a +5 class bonus and a +2 training bonus, you may add them both to the roll. Untyped bonuses are bonuses which have no type - you add all of these together when you make your roll.


Like bonuses, penalties often come from specific sources. If two or more named penalties with the same name would apply to your character, only apply the worst of those penalties. Unnamed penalties, on the other hand, accumulate without limit. Once you are ready to roll the dice, add all penalties that apply to the dice roll together and subtract that from the result.

Rewarding CreativityEdit

Players should be encouraged to attempt creative solutions to situations. Any time a situation is not perfectly handled by the rules, or any time a player's description of their attempt is particularly interesting, that player should be allowed to perform the attempt as a stunt. The DM determines an appropriate difficulty for the action based on how hard the stunt sounds to pull off, figures out a skill or other trait that is most appropriate for the player to roll, and then the player rolls their check.

Even in situations where the mechanics are well established, creative and appropriate responses to situations should be rewarded. Check modifiers or DC modifiers of -2 to +2 are appropriate in situations where player ingenuity should reasonably lean the odds in your favor. Players who find a creative and novel way of approaching a situation might even adjust the difficulty level of the task by a difficulty level or more (such as hard to moderate, or moderate to easy), which is equivalent to a +5 or higher bonus. As always, the DM is the ultimate arbitrator of what modifiers are appropriate and when.

Critical Success and FailureEdit

Most of the time, you only care if you succeed or fail. You succeed on a roll whenever the check result is equal to or greater than the difficulty; you fail on a roll if your check result is less than the difficulty. If the check result is exactly equal to the difficulty, you only achieve a basic success - you succeed, but only barely. This has no effect on gameplay, but can sometimes add cinematic flavor to the flow of the story.

Whenever you roll a natural 20, if the check result is sufficient to have succeeded at the roll, you achieve a critical success. Critical successes with attack rolls are called critical hits. When you achieve a critical success, you perform the action spectacularly well, and may optionally negotiate a relevant benefit from the DM. Certain special circumstances may allow you to achieve a critical success on a natural roll of 19, or even lower. You must still succeed at the roll (i.e., have a check value higher than the difficulty) to achieve a critical success.

If the difficulty of a check is ever higher than your total modifier + 20, you can only succeed at that task by sheer luck. If your natural die roll is a 20 before any modifiers are applied, you still achieve a basic success (but not a critical success) on that task - luck was simply on your side.

On the other hand, whenever you roll a natural 1, if the check result was insufficient to have succeeded on the roll, you achieve a critical failure. Some horrible mishap occurs, causing complications that are either specified by the task you were attempting, or up to the Game Master to resolve. Most critical failures that happen during a combat encounter will cause the critically failing character to grant advantage to all enemies until the end of their next turn. Even if your check result was sufficient to have succeeded on the roll, you still achieve a basic failure (but not a critical failure) on a natural 1 - you were simply unlucky.

Saving ThrowsEdit

Most rolls that you make are to perform a specific action. Sometimes, however, you need to make a roll to end a condition that has been inflicted upon you. These rolls are called saving throws. To make a saving throw, roll your skill, proficiency or ability modifier vs. a difficulty of 10+, unless the effect that caused the condition specifies a different save difficulty.

Common saves include:

Open Content

This material is Open Content. It is modified from [The Hypertext d20 SRDTM]. The original rules that inspired this section may be found [here].

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