Fallout 2 skills

Skill advancement
Each skill has its own skill level, which is expressed as a percentage. Skill levels can be increased by allocating skill points earned from gaining levels. Skills can also be improved by reading specific books or completing quests within the game.

Raising a skill one percentage point costs one skill point. If the player character has a specifically tagged skill, one skill point will raise the corresponding skill by two percentage points.

When purchasing skills, if the skill type is tagged, two skill % (at 1-100%) are given for each skill point spent. There are no half skill points for a cost of one. Similarly, when buying skills at cost two or higher, two or more skill points must still be purchased at once, for a cost of two, or three for three, etc. (E.g. if a skill is at 101%, and tagged, the next purchase must be of two skill, taking the skill to 103%, for a cost of two skill points.)

To raise a skill one percentage point (or two if it is tagged) when its current percentage value is between or including:
 * 1% to 100% costs 1 skill point
 * 101% to 125% costs 2 Skill points
 * 126% to 150% costs 3 Skill points
 * 151% to 175% costs 4 Skill points
 * 176% to 200% costs 5 Skill points
 * 201% to 300% costs 6 Skill points

Note that if the second of the two skill points bought for a tagged skill ends up above the transition between two costs, the cost is the lower one. For example, if a skill is at 100% and tagged, spending one skill point takes the skill to 102% for the cost of one skill point.

Skill points can be left unspent and will remain, to be spent later. This could conceivably be useful in the event of a hurdle in the game of unknown type that would prevent further progress, e.g. a locked door. Any unspent skill points in excess of 99 are lost.

Perks can be left unspent and saved until later. When the perk option opens simply select cancel and it will close. The perk window will keep reopening each time one enters the character screen but each time it can be closed again with cancel. However, one cannot stack perks; if they have avoided selecting a perk and gain another one, the first perk will be lost*.

This is no longer the case in the current version of Fallout 2 being sold on Steam and GoG. The current purchasable version has a pre-installed mod called sfall (v3.2a for Steam and GoG, v2.9a for Bethesda.net) which allows you to store multiple unspent perks without losing them.
 * Note*

Effect of stats on skills
SPECIAL stats (Strength, Perception, etc.) modify skills with a considerable bonus. The game keeps track of this bonus throughout the game, in real time. Any changes to stats, (e.g. chem effects, aftereffects, and addiction effects, the Hubologist zeta scans, Gain (statistic) perk, red memory module enhancement, mirrored shades equipped, ear bitten off by the Masticator etc.) during the game add to or subtract from this modifier.

Effect of difficulty settings on skills

 * Combat and tagged skills are unaffected by difficulty settings.
 * Active and passive skills are lowered by 10 (%) when the difficulty is changed from "normal" to "hard".
 * Active and passive skills are raised by 20 (%) when the difficulty is changed from "normal" to "easy".

Maximizing skill point gain/cost ratio
Reducing the base stat modifier of skills with chem effects, and/or by changing the difficulty to hard, can reduce the cost of skill points if it lowers the skill below a skill cost threshold. For example, Science can be raised to 135% for only one skill point per % by lowering Intelligence to zero with Mentats' aftereffects or Psycho effects (there is a limit of two chems without saving and reloading, but that will doubtlessly be necessary in any case to avoid addiction), and raising the difficulty to "hard" before spending. It can be raised a further 20% if needed, afterwards, by setting the difficulty to "easy" (same goes for reading books). This also works for higher tiers of skill point cost. Note, however, a player buying the higher tiers is doubtless in late game or finished with the game story, and may prefer to just wait for or look for the Vault City designer notes after finishing the Enclave Oil Rig.

List of skills
Note: The Fallout 2 Hintbook increases all skills to 300%. To avoid cluttering the table, it is not listed for each individual skill.

Passive skills
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