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Roller derby isn't what you remember from 20 years ago.  A lot has changed.  For one thing, the roller derby
derby has become a sport more than just entertainment.  The skaters are athletes who practice multiple
times a week, every week, all year round.
Here are the basic rules:

Each game is called a bout.  Each
bout is comprised of either 3, 20 minute periods or 2, 30 minute
periods.  Within each period are
jams.  Each jam lasts no longer than 2 minutes.  The period clock only
stops for time outs, but does not stop when a jam stops.  

There are four
blocker positions for each team.  The blocker in the front with the stripe on her helmet cover
is called the
Pivot.  The pivot's job is to maintain the speed of the pack and be the last line of defense.  The
other three blockers have no helmet cover.  Their job is to prevent the opposing team from earning points
by blocking or checking the other blockers.  Legal target zones include the arm from the shoulder to the
shoulders, below the knees, and the back.  Legal blocking zones include the arm from the shoulder to the
elbow, the torso, and the hips and booty.  Illegal blocking zones include the elbows, forearms, hands, and
the head.  Tripping is not allowed nor is blocking from behind.  Fighting will result in expulsion from the
arena.  Skaters must be in bounds when executing a block.  The eight blockers (four from each team)
comprise the
pack.

There is one
jammer position for each team.  The jammer is identified with a star on her helmet cover.  
The jammer is the only player allowed to score points.  The jammer has to make it through the pack one
time before they can earn points.  The first jammer to make it through the pack the first time without
committing any penalties is called the
lead jammer.  The lead jammer can call off the jam at any time, even
from the ground if she falls!  If she is called out to the penalty box, she loses lead jammer status. If there is
no lead jammer, the jam will go for a full 2 minutes.  After the first pass through the pack, the jammer earns
points for her team by legally passing the opposing team's blockers.  She earns one point for every
opposing team member she passes even if that team member is in the penalty box.  

Penalties are assessed as either major or minor.  Any illegal move that dramatically influences the outcome
of the game is considered a major penalty.  A skater who commits a major penalty will be sent immediately
to the penalty box.  Illegal moves that do not dramatically influence the outcome of the game are considered
minor penalties.  After four minor penalties, the skater will be sent to the penalty box.  All penalties are 1
minute long and to roll over from jam to jam and period to period.  

The team with the most points wins!

For more info/videos:

http://www.derbynewsnetwork.com/roller_derby_rules