The U.S. Constitution demands that political subdivisions have about equal populations, to maintain the "one person, one vote" standard. To achieve that, a census is conducted every 10 years. Then, soon after the national count has been completed, the redrawing of electoral lines begins.
The number of seats in the U.S. House of Representatives is set at 435, but the lines of the districts vary as the population of different areas swells and shrinks. So the first thing each census tells us is how many congressional districts every state will have for the following decade. At the height of its power, from 1945-1953, New York had 45 seats. Today it has 29, and it will lose two more in the 2012 election.
That's the simplest part. Drawing the lines to better represent the public is not.
The first rule is easy for congressional districts: They must be nearly identical in population. The rest is subject to interpretation, brutal political fights and highly complex deals.
The federal Voting Rights Act of 1965 tries to ensure that minorities not be disenfranchised via the process. Boundaries that dilute, in a variety of ways, the power of minority votes are frowned upon by the federal courts, but not always struck down when challenged.
Gerrymandering, the art of drawing warped districts to achieve political goals, happened even in the run-up to the first Congress, with Patrick Henry and the Anti-Federalists trying to tweak the 5th Congressional District of Virginia to keep James Madison from being elected in 1789. Unlike most of the gerrymanderers who would follow, they failed.
But the practice really took off in the United States after the Civil War, when districts were drawn, particularly in the South, in a way that maximized white, rural power and minimized the sway of urban, black voters.
The legacy of the practice was what the Voting Rights Act was intended to address.
CRACKING AND PACKING
In particular, "cracking," in which compact minority populations are split between districts to weaken their power, and "packing," in which large minority populations are crammed entirely into one district to keep them from controlling multiple seats, are supposed to be avoided.
New York's Senate and Assembly districts can be drawn with much more leeway in population size than congressional districts, but a population difference of more than 10 percent from the largest district to the smallest is considered suspect.
There are additional restrictions: They are meant to be contiguous in design meaning it must be possible to travel between any two points in the district without leaving the district. And compact, and set in a way that respects other political boundaries, like town, county and village lines.
It is not possible to meet these requirements perfectly, so judgments must be made, and it is here that redistricting becomes more art than science, and more politics than civics.
In the state of New York, the congressional State Senate and Assembly lines are drawn by the Legislative Task Force on Demographic Research and Reapportionment, known as LATFOR. The task force is a committee composed of one legislator and one non-legislator appointed by the president pro tem of the Senate, one legislator and one non-legislator appointed by the speaker of the Assembly, and one legislator each appointed by the minority leaders of the House and Senate.
The task force is charged with drawing the boundaries. The legislature must pass them, and the Senate and Assembly districts must be agreed upon in the same bill. The governor must then sign the new plan. If he rejects it, the legislature gets another chance. A second veto by the governor turns the process over to the courts.
If a county has an elected legislature or council represented by members elected from districts, those maps are also redrawn every decade.
Each county charter outlines its own process for creating the lines, but the composition of the districts is subject to state and federal election laws.