TTRPG Systems & Rules

Every tabletop RPG is built around one core question: when a character attempts something risky, how do the dice decide what happens? That resolution mechanic shapes everything else, from how lethal combat feels to how much the GM improvises. If D&D is the only system your group knows, understanding a few alternatives will make you a better GM even if you never switch.

Resolution Mechanics at a Glance

System	Core roll	How success works	What it rewards
D&D 5e	d20 + modifier	Meet or beat a DC; pass/fail, with advantage and disadvantage	Heroic adventure, tactical combat
Pathfinder 2e	d20 + modifier	Four degrees of success: beat the DC by 10 for a critical, miss by 10 for a critical failure	Character build depth, precise tactics
Call of Cthulhu 7e	d100, roll under skill	Regular, Hard (half skill) and Extreme (one-fifth) successes; failed rolls can be pushed at a price	Investigation, dread, fragile characters
Warhammer Fantasy Roleplay 4e	d100, roll under skill	Success Levels measure by how much you passed or failed	Gritty low-fantasy survival
Powered by the Apocalypse	2d6 + stat	10+ full success, 7–9 success with a cost, 6 or less the GM makes a move	Story momentum, character drama
Blades in the Dark	Pool of d6s, keep the highest	6 full success, 4–5 partial, 1–3 bad outcome; position and effect set the stakes	Heists, crews, consequences
Fate Core	4 Fudge dice + skill	Compare to an adjective ladder; spend fate points to invoke aspects	Narrative flexibility, any genre
Cyberpunk RED	d10 + STAT + skill	Beat a Difficulty Value; a natural 10 adds another d10, a natural 1 subtracts one	Style, gear, sudden violence

Notice the pattern: binary pass/fail systems put the drama in the numbers, while graded systems (PbtA, Blades, Pathfinder's four degrees) push the GM to narrate partial outcomes. Running a graded system for the first time, that is the skill to practice.

Choosing a System for Your Group

Ask the group these questions before anyone buys a book:

• How much combat do you want? If fights are the highlight, stay with d20 tactical games. If fights are mostly a cost, look at Call of Cthulhu or Blades.
• Do players enjoy building characters between sessions? Pathfinder 2e rewards it heavily; PbtA games give you a finished playbook in ten minutes.
• How lethal should it be? Call of Cthulhu and WFRP characters die or break far more easily than 5e heroes. That is a feature only if everyone agrees to it.
• How much does the GM want to prepare? PbtA and Blades are built around improvising from player choices; investigation games need prepared clues.
• What genre are you chasing? Systems carry tone. Running cosmic horror in 5e works against you, and a heist plays better with Blades' flashback rules than with any d20 game.

Settle the answers in a session zero, alongside safety tools and scheduling.

What Breaks When D&D Players Switch Systems

Experienced 5e players bring habits that misfire elsewhere:

• "We can fight anything." In Call of Cthulhu a single shotgun blast can end an investigator. Tell players explicitly that running is often the correct play.
• "Hit points are the tension." In Blades, harm and stress are separate tracks, and the real tension is the clock filling up. Put clocks on the table where everyone can see them.
• "Roll to find out." In PbtA you only roll when a move triggers; asking for checks out of habit slows the game. Learn the basic moves first.
• "Three actions means three attacks." Pathfinder 2e's multiple attack penalty (−5 on the second attack, −10 on the third) makes moving, raising a shield or using a skill action the better choice.

The WFRP 4e beginner's guide walks through this transition in detail for one of the most popular d100 systems.

Running a New System for the First Time

1. Start with a one-shot. A three- to four-hour scenario tests the system before anyone commits to a campaign. Most publishers offer a free quickstart with pregenerated characters.
2. Write a one-page cheat sheet. Core roll, what success levels mean, the five rules most likely to come up. Hand a copy to every player.
3. Use pregens. Character creation in an unfamiliar system eats a session. Let players play first and build later.
4. Play rules as written for the first session. House-rule only after you have seen the system work as designed.
5. Keep your setting system-agnostic. If your world notes describe factions, places and history rather than stat blocks, you can move the campaign to a new system without rewriting it. Stats are generated when needed.

Reading Path: Beyond D&D

• Warhammer Fantasy Roleplay: Beginner's Guide to WFRP 4e: careers, d100 rolls and the grim tone of the Old World.
• Warhammer 40K Factions: Complete Guide: setting background if your group wants to roleplay in the 41st millennium.
• Warhammer Chaos Gods: the four powers that drive most Warhammer antagonists.
• Fantasy vs Sci-Fi Roleplay: what changes when you move a campaign between genres.
• How to Write a One-Shot Adventure: the right format for trying a new system.
• Quest Hook Generator: hooks for D&D, Call of Cthulhu, Warhammer and Cyberpunk.
• NPC Generator: system-agnostic NPCs for any of those settings.
• Dice Roller: d4 to d100 pools with modifiers, enough for most systems in the table above.
• Digital Tools and Software Reviews: VTTs and campaign tools compared, including multi-system support.

For D&D-specific rules, homebrew and monster design, browse the rest of the DnD & TTRPG Resources Hub.

Frequently Asked Questions

What is the easiest TTRPG system for beginners?

Games built on Powered by the Apocalypse are among the quickest to learn: one 2d6 roll and a short list of moves. If your group already knows D&D, a d20 game keeps the most familiar ground.

What is the difference between Pathfinder 2e and D&D 5e?

Both use a d20, but Pathfinder 2e gives each character three actions per turn, uses four degrees of success instead of pass/fail, and offers far more character-building choices. D&D 5e is simpler and leaves more to DM rulings.

What dice does Call of Cthulhu use?

Mainly percentile dice (two d10s read as a d100), rolled under your skill value. Damage and Sanity losses use a mix of other dice, from d3 to d10 and beyond.

Are there free tabletop RPG systems?

Yes. Many publishers release free quickstart rules, Pathfinder 2e's rules are freely available on the official Archives of Nethys reference site, and systems such as Fate and Blades in the Dark publish their rules as free System Reference Documents.