One of the insights of the OSR has been the relationship between player and character, specifically as it relates to "skills" and similar systems. The idea is that the player should be able to simply describe what the character is doing, and the Referee will adjudicate the outcome based on that description. So, where a modern player might roll a character's "search" skill, the old school player would describe how the character is searching and where. This is all fine until it comes to technical skills, especially modern ones such as piloting aircraft or computer programming, where the player will either might not have any particular skill in the field or else the Referee might lack any knowledge of how that skill is employed in reality.
Top Secret approaches this situation in three different, but related, ways. The first is the easiest, and is related to the rolled abilities of the character. Things like deactivating alarms and traps, fighting, or charming a contact are covered by these. The player still describes, at least in general (but specific details are helpful, too), how the character is going about it, but the resolution is heavily influenced by the abilities of the character. Next is also easy, and those are "Areas Of Knowledge", or "AOKs". These are the most like traditional gaming skills, and cover abilities like Accounting or Chemistry. They are rated in percentages (though they can go above 100), which can be rolled against directly, or a minimum level set for success.
So far, though, we haven't seen anything about flying aircraft or forging documents (those are not covered under AOKs). Thus, in Dragon magazine and later in the Top Secret Companion, a system of "College Courses" was set out, in which the character would spend a certain amount of money and time and gain a new ability. This was not perfectly integrated, as the legacy systems of such things as martial arts (defined in the original game by the level of "Military Science/Weaponry" or "Physical Education" AOKs) were joined by the College Course system (in which one could learn various martial arts by taking a course). The idea seems to have been that someone with a lower level of Military Science/Weaponry or Physical Education could still learn the particulars of fighting with Judo, Martial Arts, Boxing, or Wrestling. Oddly, Knife Fighting and Swordplay are treated differently in the College Course system, which is something I need to think about in writing the TS retroclone. In any case, the course would also give specific increases to various AOKs or even abilities, and a modest bonus of Experience Points (though members of the Technical Bureau got quite a bit more). Here's an example of a College Course:
Time: 4 weeks
Prerequisite: Physical Strength 50+, Willpower 75+.
Areas of Specialization: Closed circuit systems, Semi-closed-circuit demand-type scuba systems.
Ability Acquired: Using semi-closed or open-circuit scuba diving equipment, the agent can dive to a maximum depth equal to the next highest fitness rating. For example, a weakling could dive to 185 feet, an average agent could dive to 285 feet, and strong and super agents could dive to depths of 380 and 435 feet, respectively. An agent can swim a distance of 5001-6000 (5000 + (1-100)x(1-10)) feet safely 85% of the time, even at the maximum depth. An agent using a closed-circuit system may dive to a depth of only 30 feet or less for 30 minutes or less. An agent can hold his breath for a number of seconds equal to his Willpower value. Increase Physical Strength and Willpower each + (1-10).
Areas of Knowledge increase: Military Science and Physical Education each + (1-10).
Credit: 60 Experience Points.
Now, there are some things in there which were not well-defined in the original game, such as the note that the agent would be safe "85% of the time" (there's no indication of what happens if that chance is failed; my guess is that it was intended as a saving throw that would automatically prevent any complications that the Referee might have considered throwing the agent's way). There are some abilities gained through College Courses, though, which seem to imply that the ability can't be performed at all by someone who hasn't taken the Course, such as piloting a space shuttle. In addition, there are many College Courses implied which were not delineated in the original material.
This is similar to the skill systems of other games in many ways, but it assumes that the player can use his knowledge to perform various actions that would be covered by skills in some modern games. For instance, the searching example I discussed above. In addition, there's a saving throw against the player's mistakes in the character's abilities, so that, for instance, a player who didn't have the character look in the right place for a particular useful object could be given a roll against the Perception tertiary trait to find it anyway. Alternately, the Referee could simply set a minimum Perception to find the item, with that coming into play if the player failed to properly direct the character.