Telepathy on Shade is also known as 'the Sight', and is much more common in females than males. Most of the documented cases have been in humans. It is extremely rare in all species, and though congenital, has not been shown to be genetically linked. Normal parents have had telepathic children, and the other way around.
Telepathy of this kind is not guaranteed to work on species not native to Shade.
There are two main aspects to the telepathic gift.
Communication is achieved by projection of a message - verbal, pictorial, auditory or other, depending on the minds involved - to a receptive person, usually (but not always) another telepath.
Reading involves plucking thoughts, and sometimes basic neurophysiological data which corresponds to emotions, from somebody's head. Non-telepaths do not know when this is occurring and cannot defend against it (with some exceptions). Just like projection, this is an active ability: the telepath has to intend to use it, and it is not always 'switched on'.
Shielding must be set up deliberately but remains in place with little or no conscious maintenance. When a telepath is trying to read another telepath's thoughts who wishes them shielded, the one with the greater skill will win. A vast skill differential would be required to read another telepath's thoughts without being detected.
Shields cannot be bolstered (more than a tiny amount) by will or concentration. Their strength depends on skill level.
Two telepaths may combine their talent to make a secure communication channel between them. In this case, the skill needed to break the security is equivalent to the product - multiplication - of the two individual skill levels. (To do so without being detected requires higher skill still.) Evidently, when three or more skilled telepaths form a secure link, it becomes very unlikely that any one individual will be able to break it.
Shade's form of telepathy is only guaranteed to be compatible with a humanoid (usually any one of the Five Races) from Shade. Most offworld species can be assumed to have sufficiently different neurobiology to render psi interactions impossible. Some compatible or (more commonly) semi-compatible offworld species have been found.
Species genetically similar to Shade's Five Races are likely to be semi-compatible; more different species are likely completely incompatible. (It's up to the two writers to decide this.)
The differential - the difference in skill levels, and not either individual's prowess - is the important factor when telepaths use their talent on each other.
Shade's telepathy is very distance-dependent. The range is reasonably constant regardless of proficiency: twenty or thirty metres. Communication over slightly greater distances (add seven to ten metres) is possible for two telepaths concentrating on each other. 'Signal boosters' are theoretically possible, but nothing of the like has been unveiled.
Occasionally there occurs a person with natural shielding but no other telepathic capabilities. This shielding is unconscious, and the subject does not know it is there (unless told), or know when somebody is attempting to break it (or succeeding). This natural shielding is roughly equivalent to that of a normal telepath with low-to-medium skill.
A system of skill 'classes' or 'ranks' has been proposed. A skill-level scale (from 0, meaning non-psi, to 200) has also been suggested. Neither of these is universally accepted or recognised, and hence they are of limited usefulness.
The skill classes are rough categories. To make sense of them, one must prefix the phrase "You could expect to find one psi of this skill in any..."
The proposed 200-point scale is designed to be a more accurate system than these classes. Where the category names fit on this scale is, naturally, a topic of contention. The 200 mark represents the absolute maximum skill level possible, and not the greatest recorded. In practice, skill levels above 160 are not seen.
Suitov of Applestone has proposed a 'negative' or 'fractional' category to accommodate those who can be broadcast to but not project or detect. He notes that a theoretical secure link formed with such an individual by a normal telepath would be easier to break than the telepath on his own.
Among the five races, the average distribution of telepathic ability (from most to least common) is:
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