General Rule

A character's skill level represents the chance of the character succeeding in a skill under "normal" stress levels. The Keeper can alter this percentage by adding or subtracting percentiles from a character's skill.

For instance, if a character is climbing an incline, the character's Climb skill may be modified by -20% if the incline is particularly steep or the footing is crumbly. A Mechanical Repair (or Locksmith, if using the optional Pagan Publishing rules) skill may be modified by -25% if the lock is particularly difficult or if the locksmith doesn't have his tools.

Likewise, skills can be modified by adding skill percentages. If a character has pitons to help climbing, a +20% modifier may be appropriate. If a lock is cheap and fairly easy to pick, a Keeper may assign a +30% chance to open it.

A player may also decide that a skill can be modified by doubling the skill percentage, or by reducing it by half or by a quarter. This is how weapon skills are altered due to range.

A skill check is allowed for all skills that succeed, even those with positive modifiers.

Specific Skill Modifiers

The following modifiers are suggested for use with specific skills.

Hide Skill

The Hide skill is modified as follows:

• Scrub is waist high grass or such grass mixed with brush. If hiding in scrub the modifiers are:
  • +5% for light scrub
  • +10% for medium scrub
  • +15% for heavy scrub
• Bush is waist high to head high brush. If hiding in bushes the modifiers are:
  • +10% for light bushes
  • +20% for medium bushes
  • +30% for heavy bushes
• Forest is woods made up of brush and/or trees that are head height or taller. If hiding in a forest, the modifiers are:
  • +10% for light forest
  • +20% for medium forest
  • +50% for heavy forest.

Sneak Skill

The Sneak skill is modified as follows:

• Scrub is waist high grass or such grass mixed with brush. If sneaking in scrub the modifiers are:
  • -5% for light scrub
  • -10% for medium scrub
  • -15% for heavy scrub
• Bush is waist high to head high brush. If sneaking in bushes the modifiers are:
  • -10% for light bushes
  • -20% for medium bushes
  • -30% for heavy bushes
• Forest is woods made up of brush and/or trees that are head height or taller. If sneaking in a forest, the modifiers are:
  • -10% for light forest
  • -20% for medium forest
  • -50% for heavy forest

Spot Hidden Skill

The Spot Hidden skill is modified as follows:

• Camouflage modifies the spot hidden roll by -20% as long as the character isn't moving or is moving slowly. A ghillie suit modifies the spot hidden roll by -40% as long as the character isn't moving or is moving slowly and the suit is worn in an area with bushes or other suitable terrain.
• Darkness modifies the spot hidden roll as follows:
  • -15% for deep shadows (e.g. night time with a full moon and/or nearby street lights)
  • -30% for normal night time darkness (e.g. night time with a partial moon showing and/or light from distant street lights)
  • -50% for near total darkness (e.g. night time with no moon and no terrestrial light source)
• Scrub is waist high grass or such grass mixed with brush. If spotting something in scrub the modifiers are:
  • -5% for light scrub
  • -10% for medium scrub
  • -15% for heavy scrub
• Bush is waist high to head high brush. If spotting something in bushes the modifiers are:
  • -10% for light bushes
  • -20% for medium bushes
  • -30% for heavy bushes
• Forest is woods made up of brush and/or trees that are head height or taller. If spotting something in a forest, the modifiers are:
  • -10% for light forest
  • -20% for medium forest
  • -50% for heavy forest

Track Skill

The Track skill is modified as follows:

• Scrub is waist high grass or such grass mixed with brush. If tracking something in scrub the modifiers are:
  • -5% for light scrub
  • -10% for medium scrub
  • -15% for heavy scrub
• Bush is waist high to head high brush. If tracking something in bushes the modifiers are:
  • -10% for light bushes
  • -20% for medium bushes
  • -30% for heavy bushes
• Forest is woods made up of brush and/or trees that are head height or taller. If tracking something in a forest, the modifiers are:
  • -10% for light forest
  • -20% for medium forest
  • -50% for heavy forest

Weapon Skills

For weapon skill modifications, see the Shooting Modifiers house rules page.

Design Notes

The rule for adding or subtracting percentages is taken from 3rd Edition RuneQuest, which uses the same system as Call of Cthulhu (the Chaosium Basic Role Play system). While never formally mentioned in the Call of Cthulhu rules, they are there in spirit. In Call of Cthulhu 5.5 Edition the chase rules have situational modifiers that add or subtract percentages from a character's drive skill. Modifiers are also found in some Call of Cthulhu scenario and campaign packs.

The Keeper is given a lot of latitude in deciding the penalty or bonus to a specific skill. This is how it's done in RuneQuest as well as other roleplaying games that allow skill modifiers due to difficulty. In general, only exceptional circumstances should have a skill modifier.