Editing Rules

===Classic Traveller===
The original Traveller rules are the standard to which all others are compared. Character creation starts by rolling 6 primary characteristics (Strength, Dexterity, Endurance, Intelligence, Education, Social Status/Standing). CT assumes you are starting as a young person with essentially no skills. Unique to Traveller at the time was the prior history system, where characters are submitted through a career (via dice rolls), gaining skills, money and equipment.

Four of the CT books, Book 4: Mercenary, Book 5: High Guard, Book 6: Scouts and Book 7: Merchant Prince include extended character generation procedures. These procedures allow players to generate a character using yearly steps, instead of four-year blocks (as in the original, basic CT character generation process). Characters from these extended systems have a higher number of more specialised skills. This process of extended character generation was included in MT.

Skill rolls are mostly based upon 2d6, roll high mechanic. Combat is based upon skills (e.g. Gun Combat skill determined your skill in combat with guns). Armour is considered to make targets harder to hit. The character's hit points are based upon the first three characteristics.

===MegaTraveller===
MegaTraveller made no fundamental changes to the character generation and core CT system, but introduced two significant additions. First was the Task System, created by [[Digest Group Publications|DGP]]. This was a systematic approach to determining and noting which skills and attributes were used for a given task, and created a regularized system of determing success or failure. The mechanics of this system continued to emulate the CT roll high system but provided a method of rating the difficulty of a task, whereas in CT this was left very much up to the Referee's discretion.

Second was the use of a variation of the Striker combat rules. With this armor absorbed damage rather than reducing the chance to hit.

===Traveller: New Era===

With TNE the game system was switched to an enhanced version of GDW's Twilight V2.2 engine for both the character generation and the task system. The enhancements where two additional attributes (Psionics and Social Standing) and a number of additional career paths. The switch was part of GDW's policy to switch all systems (2300, T2K, Twilight, Dark Conspiracy, Cadillacs&Dinousaurs, Space:1889) to the House System.

Character generation has the player roll the attributes, select a homeworld and some basic skills to represent his upbring and than send the character through various careers. Each career represented a four year period and gave a number of skilllevels, either a fixed set when taken the first time or a variable set based on the characters age with number of skilllevels received getting lower the older the character got.

For task resolution the system used

1D20 <= (Skill+Controlling Attribute) x Modifier

The Modifieres gave an estimate how difficult a given task was and ranged from Easy (x4) to Impossible (x0.25). The system was quite fast and gave a wide range of possible results while being easily calculated. Basically and attribute and skill of 5 gave a character a 50% chance to perform a Difficult task (x 0.5).

===Traveller 4th Edition===
[[Marc Miller's Traveller]] (colloquially known as "T4") was an attempt to return to the simpler, D6-only based mechanic of CT (as opposed to TNE's D6/D10 system). The inclusion of a task system within the T4 core rules imitated MT; however, the mechanic itself is a "roll under" system with both skills and characteristics used in the T4 task system when determining the target number, and the difficulty of the task determines how many dice need to be thrown (fewer for easier tasks, more for harder tasks). T4's roll under task system has both its critics and supporters; indeed, the system itself underwent a revision to remove the use of .5D within the task system.

The T4 world creation system is virtually identical to the CT system, as is the psionics system. A new form of ship design system was introduced (the Quick Ship Design System - QSDS) which, once again, returned to the simpler style of ship construction familiar to many players and Referees from CT's Book 2. However, the QSDS did not eliminate the phenomenon of broken ship designs common in CT Book 2 designs (i.e. ships with components whose total hull displacement tons exceed the allowed hull displacement tons).

The milieu of T4 returned to the beginnings of the Third Imperium, resulting in a different "feel" to the game in play for many CT/MT players and Referees.

Unfortunately T4 suffers from generally poor editing, with multiple spelling mistakes, missing tables (i.e. the Jump Drive table is missing from the QSDS section of the core rules book) and a faulty UWP generation algorithm, resulting in all worlds in the First Survey/Milieu 0 Campaign books having the same last two digits in their UWPs.

===GURPS Traveller===
[[GURPS Traveller]] uses the long established [http://e23.sjgames.com/item.html?id=SJG82-0106 GURPS 3rd edition] rules. GURPS uses a point system to buy characteristics, advantages, skills, and disadvantages. There are no random rolls for character creation. There are only four primary attributes (Strength, Dexterity, Intelligence, Health). Some of the game mechanics captured in attributes in previous version (e.g. Social Status) are represented in GURPS Traveller via "advantages". 

The GURPS skill and combat rolls are based upon rolling 3d6 and roll low mechanic. In combat, armour reduces the damage done by weapons, though some armour can make a target easier or harder to hit. Character hit points are based upon their Health.

=== GURPS Interstellar Wars ===
GURPS 4 settings, see above for more information about GURPS.
Interstellar Wars take place at the time of the Solomani expansion against an old Vilani empire. It does propose multiple campaign flavour.

===Traveller D20===
T20 uses the D20 rules estabished by [http://www.wizards.com/welcome.asp Wizards of the Coast] in their [http://www.wizards.com/default.asp?x=d20/article/srd35 System Reference Document]. D20 has a set of 6 primary attributes (T20 expands this to 8). Characters select a class which defines access to skills and advantages (called feats). T20 does have a prior history system based upon the CT model, but rolls give experience points to allow advancement through the class system. 

Skills and combat rolls use a [[D20]] (twenty-sided die), roll high against a fixed target number, modified by skill ranks plus (attribute - 10 / 2) plus other modifiers. There is a single "Combat skill" called Base Attack  Bonus (BAB), which increases based upon which class(es) the character has advanced in. T20 introduces two hit point pools called "stamina" and "lifeblood". When the first is gone, the character is unconcious, and when the second is gone the character is dying or dead. Armor both makes the character harder to hit and absorbs damage. 

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