Open Ended D10

Here I present a game mechanics idea I have for a possible game system I might do some day in the future. Things never get done until you write them down, so I decided to include it here in the wiki. You're all of course invited to use it in your own games and tweak it as you like.

I just want to take the typical d100 roll you find in Runequest (with critical hit and miss) and the open ended d100 roll in Rolemaster (with an infinite array of results) and meld them into a d10 roll. I find d100 rolls a mess because rolling two dice at the same time usually means that they scatter all around the table and floor, and they make calculations (as including modifiers or determining success) more difficult without actually adding anything in return. - Enrique Martín -

Rolling the die

You have an attribute/skill/whatever usually ranging from 1 to 8 that you need to test in a given situation. You roll one standard ten-sided die :

• If you roll a number from 1 to 9 you compare it with your attribute.
  • If you roll less or equal than your attribute you get a success.
  • If you roll more than you attribute you get a failure.
• However, if you roll a 0 (also labelled 10 in some d10 dice) you get a critical result, which can be either very good or very bad. To determine it you have to roll a second die.
  • If you roll a number from 1 to 9 you compare it with your attribute.
    • If you roll less or equal than your attribute you get a critical success.
    • If you roll more than you attribute you get a critical failure.
  • If you roll 0 again you get a double critical result. Rolling a third die will determine whether it is a double critical success or a double critical failure. Of course getting more 0's would yield triple criticals, cuadruple criticals and so on.

Properties

The open-ended d10 roll is technically equivalent to the Runequest d100 roll where a score of 1 would become a 10%, a score of 2 a 20% and so on. Everytime there is a 10% probability of getting a critical result, just in Runequest (for example, if you had a 40% you would get a critical sucess with 01-04 and a critical failure with 95-100 in your d100 roll, it is the same probability-wise).

There is also a resemblance with the Rolemaster open-ended d100 as double (1% chance), triple (1 in 1000 chance), cuadruple (1 in 10.000 chance), etc. criticals can be generated, so truly glorious rolls can happen.

Automatical success and failure rule

The die roll represents how well or bad the character fares using his existing attribute in a given situation. But sometimes external unpredictable events can determine the outcome in spite of the character effort and skill.

If a 0 is rolled (critical result), then :
• If you roll a 1 you get a lucky success regardless of your attribute score.
• If you roll a 9 you get a lucky failure regardless of your attribute socre.
• If you roll a 0 (double critical result) you can then get double lucky success by rolling a 1 or double lucky failure rolling a 9, and so on.

With this rule characters can get a success even with 0 or below attribute scores (1% chance). Masterful characters (9 or greater attribute score) can still fail (1% chance), just like in Runequest.