OSR Commentary - Overview of Frank Mentzer's B/X Dungeons & Dragons Sets Aka the BECMI edition

 Frank Mentzer did not create the original B/X (Basic/Expert) rules—those were edited by Tom Moldvay (Basic, 1981) and David "Zeb" Cook (Expert, 1981).

All photos credited to Wayne's Books 

In 1983, Frank Mentzer revised and expanded B/X into what is officially known as the BECMI edition (Basic, Expert, Companion, Master, Immortals). While gamers sometimes group Mentzer’s Basic and Expert boxes together with Moldvay/Cook as "B/X," Mentzer’s product line stretched the game from Level 1 all the way to Level 36 (and into godhood).

Mentzer's revision featured a modular, five-box structure with iconic cover art by Larry Elmore.

Set 1: Basic Rules (Red Box, 1983)

• Level Range: 1–3

• Key Focus: Introduction, solo play, basic dungeon crawling.

• Contents: Player's Manual (64 pages) and Dungeon Master's Rulebook (48 pages).

• Features: Famous for its programmed tutorial adventure featuring Aleena the Cleric and Bargle the Infamous. Mentzer redesigned this set specifically as a "self-teaching" system to guide complete beginners through character creation and combat step-by-step before opening into group play.

Set 2: Expert Rules (Blue Box, 1983)

• Level Range: 4–14

• Key Focus: Wilderness exploration, world-building, and mid-level adventuring.

• Contents: Expert Rulebook (64 pages) and module X1: The Isle of Dread.

• Features: Introduces hexcrawling, sea travel, wilderness encounter tables, and expanding campaign regional maps centered on TSR's "Known World" (later named Mystara).

Set 3: Companion Rules (Teal/Green Box, 1984)

• Level Range: 15–25

• Key Focus: Domain management, mass combat, and stronghold management.

• Contents: Player's Companion (32 pages) and DM's Companion (64 pages).

• Features: Shifts focus from traditional dungeon crawling to high-level realm management. Introduces the War Machine system for resolving mass battles, rules for building strongholds, prestige options (Paladins, Avengers, and Druids), and demispecies clan relics.

Set 4: Master Rules (Black Box, 1985)

• Level Range: 26–36

• Key Focus: Epic combat, extraplanar travel, and high-level magic.

• Contents: Master Player's Book (32 pages) and Master DM's Book (64 pages).

• Features: Introduces Weapon Mastery rules (giving martial weapons extra effects and damage scales based on proficiency), anti-magic, planar travel rules, high-level spells, and the explicit framework for mortal characters to pursue the path to Immortality.

Set 5: Immortals Rules (Gold Box, 1986)

• Level Range: Post-36 (Godhood / Immortals)

• Key Focus: Playing as divine entities and cosmic politics.

• Contents: Player's Guide to Immortals (32 pages) and DM's Guide to Immortals (64 pages).

• Features: Replaces traditional character statistics with Power Points, allowing characters who completed the Master quests to ascend into cosmic beings, managing spheres of power (Matter, Energy, Time, Thought, and Entropy) across the multiverse.

Moldvay B/X (1981) vs. Mentzer BECMI (1983–1986)

Feature	Moldvay/Cook B/X (1981)	Mentzer BECMI (1983+)
Level Cap	Level 14 max	Level 36 + Immortality
Pacing & Layout	Concise, reference-heavy, fast-paced	Tutorial-driven, step-by-step self-teaching
Thief Progression	Skills scaled faster across 14 levels	Skills stretched across 36 levels (lowering low-level success)
Advanced Systems	Simple domain rules in Expert set	Adds Weapon Mastery, War Machine, and Planar rules
Legacy Release	Standalone books	Compiled into Aaron Allston's Rules Cyclopedia (1991)

A d100 Encounter Table Tailored For classic B/X Dungeons & Dragons (and compatible Old School Renaissance systems).

 Here is a d100 encounter table tailored for classic B/X Dungeons & Dragons (and compatible Old School Renaissance systems).

Yes I totally stole this photo from Wayne's Books from here. 

To keep this highly functional for a standard B/X campaign, the table is structured around the classic dungeon tier logic: Levels 1 to 3. It balances standard low-level fantasy staples with faction dynamics, environmental hazards, and weird "OSR-style" situations that force players to interact with the environment rather than just rolling for initiative.

d100 Low-Level (Dungeon Levels 1–3) Encounter Table

d100	Encounter	No. Appearing	Activity / Current State
01-03	Acolytes (Evil)	1d8	Chanting in a low drone, carrying a sacrificial profane idol.
04-06	Amphiunder / Frogfolk	2d4	Smearing paralyzing slime onto stone tipped javelins.
07-09	Bandits	1d6+2	Setting a tripwire trap to ambush a rival dungeon faction.
10-12	Beetle, Fire	1d8	Gnawing on old torches; their glands glow with ambient light.
13-15	Beetle, Oil	1d4	Squirting blistering oil at a swarm of harmless centipedes.
16-18	Berserkers	1d4+1	Frothing at the mouth, desperately trying to break down a stuck door.
19-21	Carcass Crawler	1	Feasting on a fresh ghoul carcass; completely distracted unless approached.
22-24	Centipede, Giant	2d4	Dropping silently from the ceiling cracks onto the rearmost party member.
25-27	Crab, Giant River	1d2	Wandering up from a flooded basement level, smelling of sulfur.
28-30	Gargoyle	1	Perched perfectly still above an archway, waiting for a lone target.
31-33	Gelatinous Cube	1	Sweeping the hallway. Trapped inside it is a glowing, un-melted iron key.
34-36	Ghouls	1d4	Whispering to each other in a dead language, burying a stolen chest.
37-39	Gnolls	1d6	Argue loudly over how to split up a dead mule's hindquarters.
40-42	Goblins	2d6	Driving a small flock of blind subterranean goats toward a pen.
43-45	Gray Ooze	1	Mimicking a harmless wet puddle right in front of a treasure chest.
46-48	Hobgoblins	1d6	Marching in strict military lockstep, scouting for an invading Orc warband.
49-51	Kobolds	3d6	Retreating in absolute panic from something deeper in the dungeon.
52-54	Lizard Men	1d6	Carrying a heavy clay jar full of swamp-water and strange, squirming eels.
55-57	Lizard, Tuatara	1	Dozing near a hot steam vent; its pebble-like skin looks like a wall section.
58-60	Morlocks	1d8	Tracking the party by scent from the shadows just outside torch radius.
61-63	Orcs	2d4	Whipping a captured goblin who is forced to test a suspect floor lever.
64-66	Rat, Giant	3d6	Scurrying in a massive wave away from a sudden, localized gas leak.
67-69	Rock Baboons	1d6	Sarcastic and loud; pelting passersby with ancient, petrified dung.
70-72	Shadow	1	Following the PC's actual shadows, slowly draining the room's temperature.
73-75	Skeleton	2d4	Mechanically sweeping dust back and forth across a shattered mosaic floor.
76-78	Snake, Pit Viper	1d3	Coiled inside an empty wall alcove, agitated by the party's light source.
79-81	Spider, Crab	1d4	Camouflaged on a tapestry, waiting to drop down with a +2 bonus to surprise.
82-84	Spider, Black Widow	1	Guarding an enormous egg sac woven into a rotting wooden portcullis.
85-87	Stirges	1d10	Hanging like leathery fruit from old iron pipes, waking up as torches pass.
88-90	Thoul	1d2	Posing as a wounded, weeping human prisoner to lure someone close.
91-93	Troglodytes	1d4	Squatting in a circle, oiling their scales. The stench is noticeable 60' away.
94-96	Zombies	1d6	Shuffling aimlessly, wearing the tattered silk tunics of a long-dead cult.
97-98	Wandering Merchant	1d3	A heavily armed subterranean trader with two mutant retainers. Neutral.
99	Rival Adventurers	1d4+2	A low-level party (Fighter, Cleric, Magic-User, Thief) looking to rob the PCs.
100	The "Boss" Roll	1	Roll on the Level 4-5 table, or introduce a major local faction leader.

3 Pillars of B/X Encounter Procedures

When rolling an encounter from this table, remember to apply the classic B/X procedural checks to see how the situation actually plays out. It's rarely a straight-up fight to the death.

1. Distance & Surprise

Roll 1d6 for surprise for both the party and the encounter.

• Both surprised / Neither surprised: Encounter begins at 2d6 x 10 feet away.

• One party surprised: The unsurprised group can choose to flee, hide, or attack with a free round. The encounter begins at 1d4 x 10 feet away.

2. The Reaction Roll (2d6)

Unless the monsters are mindless undead or completely driven by hunger, roll 2d6 modified by the active PC’s Charisma to determine their initial disposition:

• 2: Immediate Attack

• 3–5: Hostile / Unfriendly (May attack, demand tribute, or mock)

• 6–8: Uncertain / Wary (Waiting to see what the PCs do; open to parley)

• 9–11: Indifferent / Friendly (Will talk, trade information, or let the party pass)

• 12: Enthusiastic / Helpful (Offers an alliance or valuable dungeon intel)

3. Morale Checks (2d6)

Monsters don't want to die. Roll 2d6 against the monster's Morale rating under two specific triggers:

1. When the first monster in their group is killed or incapacitated.

2. When the monster group is reduced to half its original numbers.

• If the roll is greater than the monster's morale score, they break and run, surrender, or beg for mercy.