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:

1. The value of your AC is determined. For any positive value (0 to 10), that number is used. For any negative value, a number between your AC and 0 is chosen at random. 10 is then added to that number.
2. The monster's level is added to that number. If the monster can't see you, subtract 2. If the monster is trapped, subtract 2.
3. If that number is less than or equal to 0, it is set to 1.
4. A random number between 0 and 20 is chosen. If the monster has more than 1 attack, 1 is added for each additional attack. (i.e. If the monster has 2 attacks, the second attack is 0 - 21, 3 attacks, 0 - 22, etc.). If the first number is higher than the random number, it is a hit. This means that the monster is less likely to hit you on each additional attack.

Examples:

1. Your AC is 7 and 10 is added to that, giving 17. The monster has a level of 1. Added together, that gives 18. On the first attack, the random number chosen is a 12. The monster hits.
2. Your AC is -5. Since it is a negative number, a number between -5 and 0 is chosen at random. In this case, it is -3. 10 is added to that, giving 7. The monster has a level of 4, and it has two attacks. Added together, that gives us 11. On the first attack, the random number chosen is 10. The monster hits. On the second attack, the random number chosen is 12 (out of a possible 21). The monster misses.
3. Your AC is -20. A number between -20 and 0 is chosen at random. In this case, -20 is chosen. 10 is added to that, giving -10. The monster has a level of 1. Added together, that gives -9. Since -9 is less than 0, it is set to 1. On the first attack, the random number chosen is 0. 1 is greater than 0, so the monster hits.

As you can see, no matter how low you get your AC, there is still a 1 in 20 chance that the monster will still hit you. This doesn't mean that you shouldn't try to get your AC as low as possible, since it still makes it harder for higher level monsters to hit you.

Any AC of negative value (-1 or lower) also decreases the damage you take.