Editing Alternity (section)

==Mechanics==

Alternity used a dual dice mechanic of rolling a control die (d20) and situation dice (such as d4) for resolving tasks but the dice size and number rolled could change due to circumstantial modifiers. The aim being to succeed by rolling under under a target number (such as a skill) then working out what degree of success or failure occurred.  

The dice used were the d4, d6, d8, d12, and d20.  The d10 was excluded.  The situation dice could be subtracted to make the roll easier or added to make it harder.  These were described as difficulty "steps", with all the factors that affect the complexity of attempting an action expressed in how many steps they add or subtract to the roll.

The use of positive and negative dice meant that there was some value to having two or three colors of dice when giving multiple players dice to roll, to represent positive, negative, and control dice.  

{| class="wikitable"
| '''Dice Actually Rolled'''
| '''Situation Dice Nomenclature'''
| '''Description'''
| '''Step Modifier'''
| '''Notes'''
|-
| d20-d20
| -d20
| No Sweat 
| -5
|
|-
| d20-d12
| -d12
| Cakewalk
| -4
|
|-
| d20-d8
| -d8
| Extremely Easy
| -3
|
|-
| d20-d6
| -d6
| Very Easy
| -2
|
|-
| d20-d4
| -d4
| Easy
| -1
|
|-
| d20
| None
| Average
| None
| Base situation dice for specialty skills.
|-
| d20+d4
| +d4
| Tough
| +1
| Base situation dice for untrained skills and broad skills.
|-
| d20+d6
| +d6
| Hard
| +2
|
|-
| d20+d8
| +d8
| Challenging
| +3
|
|-
| d20+d12
| +d12
| Formidable
| +4
|
|-
| d20+d20
| +d20
| Grueling
| +5
|
|-
| d20+2d20
| +2d20
| Gargantuan
| +6
|
|-
| d20+3d20
| +3d20
| Nearly Impossible
| +7
|
|-
|}

The add or subtract nature of dice in Alternity produced a very different probability curve from dice pooling.  Dice pools will always form a bell curve, where an add or subtract system produces a plateau where the probability of any outcome within the plateau is equal.