I've collected all the rules I've called
hidden rules whilst I was doing my annotations for OD&D. These are useful bits buried in places one would not expect to find them, thus "hidden."
Hidden Rule #1
Missile Fire Ranges: OD&D lacks any information regarding missile ranges. Or does it? My first hidden rules in the annotations contained these:
Volume II: Monsters & Magic p. 10, Manticores; the rules state (...) with the range (18”) accuracy and effect of a crossbow.
The Nixies listing on p. 15 of the same volume gives javelins a 6” throwing range.
Volume
III, Miscellaneous Weapons states the range for thrown axes, war
hammers, and possibly spears (depending upon how one parses the
description) a thrown range of 3”. All ranges are considered Medium with
regard to modifiers, no Short or Long ranges.
Hidden Rule #2
Light Sources: as with missile ranges, nothing is stated explicitly in the section regarding illumination. With a bit of reading however, the
light spells of the magic-user and cleric give us a starting point:
cast light in a circle 3” in diameter. This is somewhat less than Dr. Holmes would give a torch in his
Basic D&D "blue" book, he uses 3" radius, but it is a decent starting point.
Hidden Rule #3
Set Weapon versus Charge: under the entry
for (magical) Spears we read [a]dditionally, if something
impales itself upon the spear, damage will be double or even treble
if the force is sufficient. This
was often used as “set against charge” rule for any spear or polearm in many campaigns.
Hidden Rule #4
Partial Armor:
how often does a monster strike at an unprotected head? On this page
we find a guideline. It (the magic helm) does not protect
in the same way as Magic Armor, so if it is worn in combat any hit
upon its wearer should be given a 10% of striking the helm and
smashing it (parenthetical comment added). With a little work this could be expanded into a house-rule for partial armors, if the referee so desired.
Hidden Rule #5
Gaze Attacks:
the description for looking into this item (a mirror of life trapping) and falling victim to it
gives a very good guideline for any gaze attacks: If it is openly
displayed there is a 90% chance an unsuspecting creature will look
full within it and be trapped. There is only a 10% chance a knowing
person will be unable to avoid looking into it. Stumbling upon
Medusa unprepared gives a 90% chance of meeting her gaze, if you’re
ready you have a 1 in 10 of meeting her eyes and getting “stoned.”
Hidden Rule #6
How many adventurers can fight side-by-side in a 10' corridor? Using this rule can lead to some good PC tactics, such as using narrow passages to ameliorate a enemy's superior numbers.
Hidden Rule: most referees would
allow that hobgoblins are roughly human sized (1+1 hit dice). In the
second section of this entry, appearing on the next page, we read […]
how many [hobgoblins]
can come abreast down a typical passage in the dungeons? Allow
perhaps 3 in a ten foot wide passage, and the balance will either be
behind the front rank or fanning out to come upon the enemy by other
routes. We used this as a basis for a 3-man front line in a party
while in 10’ corridor. Accurate? It worked, and we still use that
ruling to this day. But looking back, we wonder if 3 armed and
armored fighting men side by side in a 10’ corridor would actually
work at all.
Hidden Rule #7
Rowed Ships On The Open Seas: a nearly hidden rule
allows the use of Viking longships on the open sea, as opposed to
other types of rowed vessels because they were pierced high on the freeboard.