Armor class

In Dungeons and Dragons, your armor class is a number representing your defense against attacks from monsters. NetHack borrows this concept. Because you always know your AC, you can identify the enchantment of armor by wearing it. For example, +1 armor lowers your AC by one more than normal.

A character with no armor or protection has AC 10. It is best to reduce your AC below 0. An ascension kit usually includes an AC below -20, -30, and sometimes -40.

Having a good AC is not enough to protect from some attacks; you also need to obtain resistance. In particular, reflection is a good idea.

How it works

The reason a lower armor class is better is thus:

Any number lower than 10 means the monster has to subtract that many points from its to-hit roll. A monster's to-hit roll is a number chosen at random, which is then added to any to-hit bonuses it might have. Any number greater than 0 means the monster hit.

Examples:

1. A monster gets a 15 as a random number, and has a to-hit bonus of 0, giving it a 15. Your AC is -10, meaning 20 is subtracted from the monster's roll (you have 20 more AC than the starting number, 10), giving -5. The monster misses. In this case, a monster would need a 20 or higher to hit you.
2. A monster gets 5 as a random number, and has a to-hit bonus of 3, giving it 8. Your AC is 3, meaning 7 is subtracted from the monster's roll, giving 1. The monster has hit. A monster would need a 7 or higher to hit you.

An AC lower than -10 means a monster with no to-hit bonus won't be able to hit you. That doens't happen very often, though, so you'll want to get as low an AC as possible.