Legacy is a score for "collaborative composition" or "game music."  It is also a meta-game.  The ideas involved come from Peter Suber's game Nomic, my father's description of the performance of a John Cage composition, my ex-boss' exuberant rants on Frippertronics, and my lover's description of John Zorn's COBRA and Ed Chang's Map scores.  There are no winners, losers, points, or grades, although the competitively minded could add such rules easily either before or during the course of play.
To play, each player needs access to a recording studio with a multi-track recorder.  Two or more players may use the same studio.  Each studio's multi-track recorder must be fully media compatible.  Before the beginning of the game, players draw lots to determine who will be the next player after them, such that all players are arranged in a single cycle.

Also before the beginning of the game, a blank piece of media, a blank track list, and a copy of these rules are prepared.  The media is a single piece of media for the multi-track recorder used by all players.  For example, if all players were using an ADAT, the media would be one ADAT tape cartridge.  The rules are a textual representation of the rules of the game in a language all players are fluent in.  It is probably best to distribute the rules as an ASCII (Unicode?) text file so that they can be modified easily using a broad variety of computer workstations.  The track list describes the contents of each track on the media.  Each track must be labeled as new, last, or legacy on the track list.  Labeling tracks this way makes the rules clearer, also making future rules easier to write.

At the beginning of the game, all tracks on the track list should be marked as legacy.  Players draw lots to determine who goes first.  The first player then receives the media, track list, and rules.  The initial rules of the game (with subjective commentary to the right) are given below.

This set of rules is seeded from Legacy Rules 1.1. If the initial list of rules evolves and new games are begun, this rule helps in documenting what came from where.

Legacy Rules 1.0 was written in September 1999.   No games were played with rules 1.0, which were discarded in favor of this slightly modified version 1.1.

No rule may be deleted or modified. This is why the game is called Legacy.  You must operate under all previous restrictions.
All rules are in effect at all times and under all circumstances unless explicitly stated otherwise.
The game is over when a player's turn begins and that player cannot follow the rules.
A players turn begins when he or she receives the media, rules, and track list. The rules define beginnings and ends of turns for the sake of future rules.
A players turn ends when he or she sends the media, rules, and track list to the next player. Note that rules regarding how long a turn may take, or how much time may pass between turns, is left indefinite. 
During his or her turn, the player must record new material over one or two legacy tracks, simultaneously marking these tracks as last on the track list.  This action is called injection. Different styles apply here.  It is probably best to record something that goes well with the tracks marked new, and that makes a small change in the overall mix.  For example, add a snare track instead of an entire percussion section. 

Note that the recording and marking of tracks is required to be "simultaneous".  This is to prevent future rules from modifying this action.

Player actions are named to make it easier to write future rules.

After injection, the player must mix all last tracks together into one or two legacy tracks.  Simultaneously, these resulting tracks are marked as new.  Optionally, legacy tracks may be simultaneously incorporated into the new track.  Some portion of all last tracks must be audible to the player in the new track.  This action is called combination. Note that mixing last tracks and legacy tracks all must happen at once.  The rule is worded to make it difficult or impossible to add future rules that change the way last/legacy tracks are combined.
After combination, the player reclassifies all last tracks as legacy tracks.  Immediately after that, the player reclassifies all new tracks as last tracks.  This action is called reclassification.
After reclassification, the player adds one or more new rules to the list of rules.  This action is called extension. Generally compositional rules are added, but there is no restriction to the nature of the rules you may add.  For example, a player might add a rule about making future rules, or a more restrictive version of an existing rule.

Note that any player may end the game at any time by making a rule that cannot be followed.

A players turn may not end until after injection, combination, reclassification, and extension.
Here is an example of play.  It may help to follow along with your own track sheet.

The players are John, Paul, and  George.  They draw lots and determine that the order of play will be John, Paul, George, then Ringo.  A blank ADAT tape, track sheet, and floppy with a copy of the initial rules are prepared and given to John, which begins his turn.

The rules as they are sketch out several things that John must do, and a rough order in which to do them.  One of the rules says that he "must record new material over one or two legacy tracks simultaneously marking these tracks as last on the track list." All of the other rules that demand that John do something say he must do something else first, so John toys with a short wave radio and small analogue modular for a time and comes up with a brilliant track featuring barely audible instructions for breeding Moose.  It is a mono track, and all of the tracks are currently marked as legacy, so John can record this track onto any track on the tape.  He uses track 1 and marks that track last on the track sheet.  John reads ahead and sees the rule for extension. He really wants to add rules to the list now, but the rules stipulate that he can't do extension until after reclassification, and he can't do reclassification until after combination.  John does combination by copying his last  track onto another legacy track naming the copy track (track 2) newReclassification must be done next, so John reclassifies track 1 as legacy and track 2 as last.  Finally, John can add rules in an act of extension.    He decides to add a traditional composition rule, "Acceptable themes for tracks are hair, horns, and antlers.  No narrative tracks are acceptable."  He also adds a technical rule, "Any animal sounds used must be the sounds of humans or moose."  John thinks he has now effectively restricted the work forevermore in such a way that it must at least skirt the domain of silliness, and so sends off the tape, track list, and new rules to Paul.

Paul gets the tape, track list, and rules.  The moose stuff doesn't really work for him (being a satanist, he'd have preferred a goat), but he's still game.  First he has to do injection.  He records a simple little bass synth loop into track 3, which he then marks as last.  He notes on the track list that the track is an abstract expression of the DNA/RNA program giving rise to moose antlers.  John's rule is subjective and vague enough to allow this.  Next, Paul must do combination.  He mixes tracks 2 and 3 onto track 4, making sure that both are audible but that the sillier parts of  John's track 2 are absent.  Paul marks track 4 new.  Next, reclassification.  Paul reclassifies tracks 2 and 3 as legacy and track 4 as last.  Finally, extension.  Paul wants to move the piece away from silliness and more toward his pet issue, Satanism.  First he considers adding a rule saying "Every track must be an act of worship of Satan."  That would make him no better than John, however, and would most likely cause even more reactionary rules from George.  Really, he decides, he just wants to get away from the moose thing.  He adds the rule "Any animal sounds used must be the sounds of humans.  An animal sound is a sound made by forcing air over specialized sound making organs of the body."  He thinks that should block the moose nonsense.  He considers restricting John's "acceptable themes" rule to mythological creatures with horns, but decides that the original rule is so subjective as to be meaningless anyway.  He sends off all the stuff to George.

George gets right down to business.  For injection he cooks up a very hissy stereo high hat loop that syncs with Paul's track.  He records it to track 7 and 8 and marks those tracks last.  Note that he could have used any tracks but track 4.  On the track sheet, he simply puts ditto marks under the explanation given for Paul's tracks, mocking John's rule even further.  For combination he mixes tracks 4, 7, and 8 into tracks 5 and 6, marking tracks 5 and 6 as new.  For reclassification he marks tracks 4, 7, and 8 as legacy and tracks 5 and 6 as last.    To prevent future stupidity on the part of rule makers, George introduces the following rule.  "Before extension, the player writes down some rules.  Only these rules from this writing that have been approved by at least 3/4 of the other players may be entered in extension.  Writing the rules and measuring approval for each rule is called voting, and considered to happen simultaneously."  Furthermore, he imagines John will try some stupid ballot box stuffing stunt, so he goes ahead and adds another rule.  "New players may not be added to the game."  Note that George is not subject to his own rules until his next turn.  George decides to add some actual musical content into the rules, rather than just rules about rules, so adds the following rule (borrowed from Brian Eno).  "If possible, play one staccato note on every eighth note in every measure."  Although this rule is as easy to ignore as John's rule about hair, horns, and antlers, George hopes that John and Paul will find it interesting and try it anyway.  With no more rules to add, George sends all the stuff on back to John.

And so on.  The possibilities are still limitless even though some fairly great restrictions have been placed on the music in just the first three turns.  Perhaps that is the point of the exercise.