Here's my combat sequence house rules from my original D&D campaign. It starts off the full combat sequence, but at the end there is the "simplified sequence." I often default to the using the "simplified sequence," but with rulings informed by my familiarity with the details of the full sequence.
# Combat
A round of combat is approximately 10 seconds of time.
## Surprise
At the start of an encounter, each side checks for surprise, if applicable.
The standard chance of surprise is 2 in 6, as follows:
Die Roll (d6) Result
--------------- -----------------------------------------
1 Complete surprise (enemy gets 2 actions)
2 Surprise (enemy gets 1 action)
3-6 Not surprised
Circumstances may modify the chance of the surprise. For example, a certain
monster may surprise with a 3 in 6 chance. In such cases, a die roll of 1
indicates complete surprise, and a die roll of 2 or 3 indicates surprise. In
other words, only a roll of 1 indicates complete surprise.
### Surprise Actions
Surprise actions are free actions which occur (or start) before the regular
melee round begins. Examples include movement, firing a loaded crossbow, melee
attacks, et cetera. Spells cast as a surprise action take effect as normal,
according to the rules for casting times (e.g., 1st-2nd level spells, power
words, and item activations would take effect immediately; higher level spells
cast as a surprise action would take effect during the normal phase of the
melee round). As always, the caster may only cast one spell during the round,
even if it was cast as a surprise action.
Movement taken during surprise is a maximum of a 1/4 move for surprise and a
half move for complete surprise.
Archers may fire one arrow per surprise action. Crossbowmen and slingers may
only fire once during surprise, even if they have two surprise actions. If a
character has multiple hurled weapons in hand, one weapon may be hurled in
each surprise action.
## Encounter Distance
### Underworld/Dungeon/Indoors
Standard encounter distance is 20-80 feet (2d4). However, if surprise is
indicated for either side, then encounter distance is 10-30 feet (1d3).
The DM may adjust encounter distance according to the circumstances.
### Wilderness/Outdoors
Standard encounter distance is 40-240 yards (4d6). However, if surprise is
indicated for either side, then encounter distance is 10-30 yards. The DM
may adjust encounter distance according to the circumstances.
## Movement Rates in Combat
In a combat round, a character's full move is his standard movement rate
divided by three. A charge doubles that.
Base Move Combat Move Charge
----------- ------------- --------
24 80' 160'
21 70' 140'
18 60' 120'
15 50' 100'
12 40' 80'
9 30' 60'
6 20' 40'
3 10' 20'
## Combat Sequence
1. _Initiative_: Both sides roll 1d6. High roll has initiative.
2. _Missiles & Spells_: In initiative order, both sides fire missiles,
cast spells, et cetera.
3. _Movement (Side A)_: Side with initiative moves up to half move.
4. _Movement (Side B)_: Side without initiative moves up to half move.
5. _Missiles & Spells_: In initiative order, both sides fire missiles, cast
spells, et cetera.
6. _Movement (Side B)_: Side without initiative moves up to half move.
7. _Movement (Side A)_: Side with initiative moves up to half move.
8. _Missiles & Spells_: Unengaged combatants fire missiles, cast spells, et
cetera.
9. _Melee_: Engaged combatants fight one round of melee.
### Rules Governing Missiles/Ranged Attacks
* Archers standing still may fire twice (in phase 2 or 5, and in phase 8 if the
archer remains unengaged).
* Archers taking a half-move may fire once (in phase 2 or 8). However, archers
with split-move-and-fire ability (e.g., elves, mounted archers) that take a
half-move may fire once in a missile phase of their choice (2, 5, or 8).
* Archers taking a full move may not fire. However, archers with split-move-and-
fire ability (e.g., elves, mounted archers) may take a full move and fire
once, in phase 5, only.
* Slingers standing still may fire once (in phase 2, 5 or 8).
* Slingers taking a half-move may fire once (in phase 2 or 8).
* Slingers taking a full move may not fire.
* Crossbowmen standing still may fire once (in phase 2 or 5) and reload*.
* Crossbowmen standing still may reload* and fire once (in phase 5 or 8).
* Crossbowmen taking a half-move may fire once (in phase 2 or 8) or reload*.
* Crossbowmen taking a full move may reload*, but may not fire.
* Combatants hurling spears, axes, or hammers may fire once (in phase 2, 5, or
8) and take a full move, including charge, if desired.
* Combatants hurling daggers or javelins may fire once (in phase 2, 5, or 8)
and take a full move, including charge (and melee), if desired.
* Combatants hurling daggers or javelins may fire twice (in phase 2 or 5, and
phase 8) and take a full move without charge (no melee attacks allowed).
* Combatants hurling in-hand weapons without moving may fire once and still
engage in melee attacks if charged (but may not set spears or similar
weapons against a charge).
* Heavy crossbows require a full round to reload, and no movement is allowed
while reloading.
### Rules Governing Spells
* Spells may be cast as "spells," from "item activation" (e.g., wands), from
scrolls (this is not considered an item activation), or from innate spell
abilities (due to race, et cetera).
* With the exception of power words and item activations, a spell caster cannot
move and cast a spell in the same round. Power words and item activations
can be combined with up to a full move without charge, and may be cast in
phase 2, 5, or 8.
* With the exception of power words, item activation, and spells designed to
be cast in melee, spell casters may not cast a spell while engaged in melee.
* If a caster is engaged in melee or takes damage after a spell is commenced
but before it is finished, the spell is interrupted and lost.
* A spell caster may cast a maximum of one spell per round.
* Casting time depends on the level and type of spell.
Spell Casting Time
-------------------------- -------------------------------
1st-2nd level spells Immediate
Power Words Immediate
Holy Words Immediate
Word of Recall Immediate
Item activation Immediate
Innate spell abilities Immediate*
3rd-6th level spells +1 Spell Phase
1st-2nd level scrolls +1 Spell Phase
7th-9th level spells Full Round
3rd-6th level scrolls Full Round
7th-9th level scrolls Full Round + 1 Spell Phase
* unless otherwise specified by the ability being used
### Rules Governing Melee & Movement
* Combatants who take a full move may not make any melee attacks unless they
charge, even if they move into engagement range.
* A charge into engagement gives the charging combatants a +2 to hit in the
initial round of melee. Charging characters suffer a -1 penalty to AC in the
initial round of melee (this does not apply to ranged attacks against them).
When combatants are charging, weapon length decides who strikes first (rather
than initiative).
* Only one charge may be taken per turn (not per melee round). This probably
means only one charge may be taken in most melee engagements.
* Combatants are considered engaged in melee when the distance between them is
equal to or less than the longest reach or 10 ft, whichever is larger.
* A moving combatant who becomes engaged may not leave engagement or continue
movement to the flanks or rear of his opponent during the initial round of
engagement. A combatant not already engaged in melee may move a maximum of 5'
right or left in order to confront and contact an enemy attempting to bypass
or move into a flanking position.
* If one combatant in a melee has allies to his immediate left or right, and
these allies are not engaged with other enemies, the allies may move into
flanking positions against their common enemy after the first round of melee.
* Flanking position grants a +1 bonus to hit and negates any benefit from the
target's shield.
* Rear positioning grants a +2 bonus to hit and negates any benefit from the
target's shield.
* Flee Melee: an engaged combatant may flee melee at full movement, but he
loses the benefit of his shield and engaged enemies get a free attack with a
+2 bonus to hit.
* Retreat: an engaged combatant may retreat from melee at half-move. Non-elite
combatants (applies only to NPCs) must make a morale check when retreating.
Retreating combatants may not attack in the round they move, and they end
their movement still facing their enemies.
* Fall Back: an engaged combatant may fall back while fighting. Melee proceeds
normally, but only 10 ft. of movement is allowed.
## Simplified Combat Sequence
1. Both sides roll 1d6 for initiative; high roll wins.
2. Winning side fires missiles, starts spells (and finishes spells of level 1-2)
3. Losing side fires missiles, starts spells (and finishes spells of level 1-2)
4. Both sides move
5. Spells that were started in 2-3 take effect (if they weren't completed
earlier); archers who didn't move and haven't been engaged in melee may fire
again.
6. Melee
The DM may use the details of the full sequence to inform rulings made when
using the simplified combat sequence.
When I run AD&D, I use my own interpretation of the 1e AD&D initiative rules. I've also used my original D&D sequence, above, with AD&D. That works fine, but when I'm running AD&D I prefer to keep the rules on segments, etc., so I usually stick to my interpretation of "BTB."