HOUSE RULES: Combat Mechanics (AC and THAC0)

Version 1.0, published on 29 May 2010. Written by Andrew.

Background

Until further notice Andrew will be using a new AC and THAC0 system based on the 3rd Edition rules. The purpose of this change is to speed up combat.

New Armour Class (higher is better)

New Armour Class = 10 (base) + armour bonus + shield + other modifiers

To convert from an old AC: New Armour Class = 10 + (10 – old AC)

Armour

Armour Bonus

Leather

2

Studded Leather

3

Ring Mail

3

Scale Mail

4

Chain Mail

5

Splint Mail

6

Banded Mail

6

Plate Mail

7

Field Plate

8

Full Plate

9

Shield

1

New Attack Roll (higher is better)

Attack Roll = 1d20 + class attack bonus + other modifiers

If Attack roll >= new AC then you hit.

A 1d20 roll of “1” is always a miss.

To convert from an old THAC0: Attack Bonus = 20 – old THAC0 (see tables below )

Notes:
                1. Not using size modifiers from 3rd edition at this stage.
                2. Still using the old THAC0 table converted, not using the new Class Attack Bonus tables from 3rd Edition

New Class Attack Bonus Table – Based on Second Edition THAC0

 

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

Priest

0

0

0

2

2

2

4

4

4

6

6

6

8

8

8

10

10

10

12

12

Rogue

0

0

1

1

2

2

3

3

4

4

5

5

6

6

7

7

8

8

9

9

Warrior

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

Wizard

0

0

0

1

1

1

2

2

2

3

3

3

4

4

4

5

5

5

6

6

Monster

1

1

3

3

5

5

7

7

9

9

11

11

13

13

15

15

17

17

19

19

Second Edition THAC0 Table

 

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

Priest

20

20

20

18

18

18

16

16

16

14

14

14

12

12

12

10

10

10

8

8

Rogue

20

20

19

19

18

18

17

17

16

16

15

15

14

14

13

13

12

12

11

11

Warrior

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

Wizard

20

20

20

19

19

19

18

18

18

17

17

17

16

16

16

15

15

15

14

14

Monster

19

19

17

17

15

15

13

13

11

11

9

9

7

7

5

5

3

3

1

1

Comparison of new and old armour class and attack systems

The following example clearly shows that caclulating if a charcter hit something is easier using the new system.

Old System:

Q Featherhammer (THAC0 = 16, +2 Str, +1 Axe) wants to hit Cadra (AC=1).

He rolls a 14, so he hits AC: 16 – (14 + 2 + 1) = -1, which is <= 1, so he hits.

New System:

Q Featherhammer (AB = +4, +2 Str, +1 Axe) wants to hit Cadra AC=19).

He rolls a 14, so he hits AC: 14 +4 + 2 + 1 = 21, which is >= 19, so he hits.

Surprise and Initiative (same as I have used before)

Surprise: One roll and result for all involved.  Roll 1d10, surprised on a 1 or 2, use worst dexterity modifier.

Initiative: Roll 1d10, winner goes first in each phase: Fast, Average, Slow (V Fast and V Slow used when needed).
Simultaneous Initiative in Combat and Tactics is used.