Being trained in a Knowledge skill represents having studied some body of lore, possibly an academic or even scientific discipline. There are 10 Knowledge skills, each with it’s own typical fields of study:

• Arcana (ancient mysteries, magic traditions, arcane symbols, constructs, dragons, magical beasts)
• Dungeoneering (aberrations, caverns, oozes, spelunking)
• Engineering (buildings, aqueducts, bridges, fortifications)
• Geography (lands, terrain, climate, people)
• History (wars, colonies, migrations, founding of cities)
• Local (legends, personalities, inhabitants, laws, customs, traditions, humanoids)
• Nature (animals, fey, giants, monstrous humanoids, plants, seasons and cycles, weather, vermin)
• Nobility (lineages, heraldry, personalities, royalty)
• Planes (the Inner Planes, the Outer Planes, the Astral Plane, the Ethereal Plane, outsiders, planar magic)
• Religion (gods and goddesses, mythic history, ecclesiastic tradition, holy symbols, undead) Check

How do I use this skill? Answering a question within your field of study has a DC of 10 (for really easy questions), 15 (for basic questions), or 20 to 30 (for really tough questions).

In many cases, you can use this skill to identify monsters and their special powers or vulnerabilities. In general, the DC of such a check equals 10 + the monster’s CR. For common monsters, such as goblins, the DC of this check equals 5 + the monster’s CR. For particularly rare monsters, such as the tarrasque, the DC of this check equals 15 + the monster’s CR or more. A successful check allows you to remember a bit of useful information about that monster. For every 5 points by which your check result exceeds the DC, you recall another piece of useful information.

What type of action do I take? Usually none. In most cases, a Knowledge check doesn’t take an action (but see “Untrained,” below).

Can I try this again? No. The check represents what you know, and thinking about a topic a second time doesn’t let you know something that you never learned in the first place.

Using this skil untrained: You cannot make an untrained Knowledge check with a DC higher than 10. If you have access to an extensive library that covers a specific skill, this limit is removed. The time to make checks using a library, however, increases to 1d4 hours. Particularly complete libraries might even grant a bonus on Knowledge checks in the fields that they cover.