TEXT: OSR
  Rosetta Stone
  
  Table Key
  D20: This is the D20/3.5/Pathfinder skill system. It assumes constantly scaling bonuses and difficulties.
  5th (DC): This is the 5th edition “bounded accuracy” system. It assumes a maximum proficiency bonus of +6, a maximum statistic 
  bonus of +5, giving your average 20th level character a +11 on rolls. Expertise can double the proficiency bonus and magic can add up 
  to +3, making the maximum normal value a character can achieve +20 on a roll. Characters, particularly rogues, will hit this range. If 
  your rogue has a passive perception of 29, the expectation is they will see all traps. Keep this in mind when converting difficulties. 
  C&C: This is the challenge class rating for Castles & Crusades. The odds are calculated assuming primes. If not prime, simply subtract 6 
  to get the same chance for a non-prime. A prime stat has a challenge value of 12 for checks, a non prime 18. Challenge class is added 
  to the base (prime/non-prime) difficulty. Statistic bonuses range from -4 to +3, your level is added if applicable to the check.
  ACKS Throw: This is the player facing value the roller must succeed against in the Adventurer Conqueror King system. Modifiers are 
  applied to the die roll. You may alternately align these numbers with the Base Success Chance value, but the proficiencies make it clear 
  that average tasks succeed on a 11+, where as formidable tasks (using healing to cure disease for example) still have a chance of 
  success.
  D100: This refers to the modifier applied to the target number on any system using a roll under % system for skills, such as Hackmaster 
  4th edition. For open ended percentile systems like Rollmaster, these can modify the percentile result (but if they modify the result over 
  95% or under 5%, they will not cause the roll to become open ended.)
  NWP: This is the modifier applied to Dungeons & Dragons 2nd edition Non-Weapon Proficiency system. These modifiers are applied to 
  the target number. Modifiers for non-weapon proficiency rise slower because it is much more costly to become more effective at those 
  skills (+5% every 3-4 levels). This makes the approximate success chance for non-weapon proficiency inaccurate, but does keep it in 
  line with the difficulty of the task. Non-weapon proficiency modifiers should probably cap out around -8. I have never played a 2nd 
  edition game where the house rule of each time you spend a non-weapon proficiency slot on the skill, it increase the bonus to the roll 
  by +4 instead of +1.
  BSC: This is the base success chance. It is simply a representation of the percentage chance of success at any task in the abstract. 
  Consider this the baseline of the table for comparison.
  D6: Using a system like Skills: The Middle Road or Lamentations of the Flame Princess? This is a d6 representation of core chances for 
  success.
  Difficulty of Task: This is a word description of the difficulty of the task being accomplished, like the base chance of success, 
  intended to act as a baseline.
  MonsTer Conversions
  Original Dungeons and Dragons: Monsters all use D6 for hit dice.
  Basic Dungeons and Dragons/ACKS: Monsters use D8 for hit dice, but weapon damage and player hit point totals remain at 
  OD&D levels, making monsters much more deadly.
  1st/2nd Edition/C&C: Monsters use D8 for hit dice, but player hit point totals are higher. C&C requires armor class conversion. 
  All the above versions of D&D use a +X modifier following hit points to indicate that it hits “above it’s class” gaining a bonus to ThAC0.
  3rd Edition: Monster hit points vary by monster type and have Constitution bonuses applied. When converting back to earlier 
  editions, eliminate the Constitution bonus and roll the appropriate die for hit dice. When converting forward use the appropriate die 
  type for the monster type and consider increasing hit points based on hit die due to increased player damage output. Armor classes 
  must also be capped if converting backwards, eliminating dexterity bonuses and adjusting for scaling increases.
  Hackmaster 4e: This is functionally equivalent to 1st edition if the hit point kicker is removed.
  5th Edition: 5th edition monsters are the most difficult to convert. The difficulty is inflated hit point totals and bounded accuracy. Here 
  are some guidelines for on the fly conversion.
  •	 Ability Scores—Use ½ Fortitude, Reflex and Will saves for ability bonuses. 
  •	 Alternately the bonus is +½ CR. 
  •	 Proficiency bonus is as Character level to Hit Dice, i.e. 1-4 Hit die is +2, 5-9 Hit die is +3.
  •	 To hit bonus—is ½ CR + Proficiency Bonus, no penalties for multi-attack, damage should remain the same
  •	 Skills—Use ½ appropriate save bonus + proficiency bonus. Double this bonus if trained.
  •	 Armor Class—Is bounded. Use AC 10-12 for lightly armored, AC 13-15 for medium armor, and AC 16-19 for heavy armor.
  •	 Hit points—CR×10+20. If not, adjust up only.
  •	 Abilities—keep and use the rules for 5th edition, i.e. Magic Resistance gives advantage on saves.
  Baseline Assumptions
  Approximate % chance of success is for a 1
  st level character who has the skill in question. For each system this assumes different 
  baselines.
  •	 D20 is 1 rank + 3 class skills + 4 bonus in stat = +8
  •	 C&C assumes 1 level +1 bonus from stat = +2
  •	 D100 assumes 25% mastery at 1
  st level
  •	 NWP assumes a base stat of 14 in the non-weapon proficiency
  •	 5th assumes a +2 proficiency and +3 statistic bonus = +5
  I selected these values because they are average baseline character creation values for core stats for each system in question, (stat of 
  18 for point buy D20, stat of 14 for roll 4d6 in order drop lowest, and 25% for taking a skill multiple times in a system like Hackmaster 
  4
  th edition).
  There are a lot of OSR type games.
  Each game features a unique variant of some basic math. Each has a few assumptions about how these numbers play and scale, and 
  each edition carries its own variation affecting play. Converting between systems is trivial. However, for some of the subsystems, such 
  as converting 1st edition style non-weapon proficiencies to 3
  rd skill DC’s, or 5
  th edition skills back to 3
  rd edition skill DC’s it can get a little 
  more complicated. 
  A + always means that the skill is easier. In roll under systems (D100/NWP) this means the modifier is applied to the target number.
  20-ThAC0 = BAB
  20-BAB = ThAC0
  20-Descending AC = Ascending AC
  20-Ascending AC = Descending AC
  D20 (DC)
  5th (DC)
  C&C (CC)
  ACKS throw
  D100
  NWP
  ~% BSC
  D6
  Difficulty of 
  Task
  10
  5 DC
  0 (-6)
  4+
  +65%
  +4
  90%
  5 in 6
  Trivial-
  11
  10 DC
  0 (-5)
  5+
  +60%
  +4
  85%
  5 in 6
  Easy
  12
  10 DC
  0 (-4)
  6+
  +55%
  +3
  80%
  5 in 6
  Easy
  13
  10 DC
  0 (-3)
  6+
  +50%
  +3
  75%
  5 in 6
  Easy
  14
  10 DC
  0 (-2)
  7+
  +45%
  +2
  70%
  4 in 6
  Average
  15
  15 DC
  0 (-1)
  8+
  +40%
  +2
  65%
  4 in 6
  Average
  16
  15 DC
  0
  9+
  +35%
  +1
  60%
  4 in 6
  Average
  17
  15 DC
  1
  10+
  +30%
  +1
  55%
  4 in 6
  Average
  18
  15 DC
  2
  11+
  +25%
  0
  50%
  3 in 6
  Average
  19
  20 DC
  3
  12+
  +20%
  -1
  45%
  3 in 6
  Difficult
  20
  20 DC
  4
  13+
  +15%
  -1
  40%
  3 in 6
  Difficult
  21
  20 DC
  5
  14+
  +10%
  -2
  35%
  2 in 6
  Difficult
  22
  20 DC
  6
  15+
  +5%
  -3
  30%
  2 in 6
  Difficult
  23
  20 DC
  7
  15+
  +0
  -4
  25%
  2 in 6
  Difficult
  24
  25 DC
  8
  16+
  -5%
  -5
  20%
  1 in 6
  Very Difficult
  25
  25 DC
  9
  16+
  -10%
  -6
  15%
  1 in 6
  Very Difficult
  26
  25 DC
  10
  17+
  -15%
  -7
  10%
  1 in 6
  Very Difficult
  27
  25 DC
  11
  17+
  -20%
  -8
  5%
  1 in 6
  Very Difficult
  28
  25 DC
  12
  18+
  -25%
  -9
  0%
  0 in 6
  Formidable
  29
  25 DC
  13
  19+
  -30%
  -10
  -5%
  0 in 6
  Formidable
  30
  30 DC
  14
  20+
  -35%
  -11
  -10%
   0 in 6
  Impossible+
  31
  30 DC
  15
  20+
  -40%
  -11
  -15%
   0 in 6
  Impossible+
  32
  30 DC
  16
  21+
  -45%
  -12
  -20%
   -1 in 6
  Impossible+
  33
  30 DC
  17
  21+
  -50%
  -12
  -25%
   -1 in 6
  Impossible+
  34
  — DC
  18
  22+
  -55%
  -13
  -30%
   -1 in 6
  Godlike+
  35
  — DC
  19
  22+
  -60%
  -13
  -35%
   -1 in 6
  Godlike+
  Using This Table
  So you’re reading a module and you come across a DC 22 perception check to locate a trap. This translates to a +5% on a Find Traps 
  or Observation roll, or a 2 in 6 chance of locating the trap. Simple, eh?
  Playing 2nd edition and want to know if your alchemist can produce that DC 20 alchemist fire? Use that Alchemy NWP at -1 to find 
  out!
  
SOURCE: textdata/thevault/Dungeons & Dragons [multi]/5th Edition (5e)/3rd Party/Hack & Slash/OSR Rosetta Stone.pdf