The Volcker Rule, a creation of the 2010 Dodd-Frank financial regulation overhaul, faced its last step before taking effect on Tuesday, when five federal regulatory agencies voted on its approval.

That it took five agencies, with a total of 22 voting members, to approve the law is a reflection of the way regulation works in the U.S. and a hint to understanding why it took three years for regulators to finish writing the rule.

The Volcker Rule would ban banks from proprietary trading — that is, buying and selling of securities and derivatives for their own profit.

Enforcing the ban means monitoring a number of different institutions and financial activities. Instead of one overarching financial regulator, the U.S. has a host of agencies because historically the government has created new regulatory agencies to respond to new threats arising in the financial system. As a result, five of the biggest agencies were required to jointly write the Volcker Rule, which is complex and touches almost every aspect of finance.

The five:

• The Federal Reserve, the central bank of the U.S., is the most important regulator. It has the power to lend to banks during panics and accordingly is responsible for regulating those banks. Following Dodd-Frank, it also monitors any business it considers too-big-to-fail, and is tasked with ensuring the safety of the financial system as a whole.

• The Office of the Comptroller of the Currency, part of the Treasury Department, was created in 1863 to supervise federally chartered banks and to introduce a national currency (previously, currency was issued by state-chartered banks). It still oversees national banks today.

• The Federal Deposit Insurance Corporation was founded at the depths of the Great Depression in 1933 to provide insurance to depositors to prevent runs. It oversees banks that receive deposit insurance. Dodd-Frank tasked the FDIC with liquidating big banks in the case of failure.

• The Securities and Exchange Commission, another New Deal creation, oversees securities trading and markets to prevent fraud.

• The Commodity Futures Trading Commission monitors derivatives trading, such as options, calls and puts. It was created in 1974 as finance started to make more use of complex derivatives, and was given the power to oversee far more of those transactions by Dodd-Frank.

Section 619 of the Dodd-Frank law ordered all five agencies to work together in writing the various parts of the ban on proprietary trading to make sure that the rules did not conflict or overlap.

One positive to having so many agencies involved in banking oversight might be redundancy: If regulators in one office are negligent, others may pick up something they missed.

But there are also problems to worry about with having so many cooks in the kitchen. One is regulatory arbitrage: Firms trying to place a line of business or their entire company under the jurisdiction of an agency they see as lax.

Another is turf-protecting. An enduring feature of government agencies is that their staffs are reluctant to give up any power delegated to them. When the government tries to reorganize the structure of the regulatory system, as it did with Dodd-Frank, agencies work to protect their turf, and ensure that they don't lose any authority to another agency, which might use that authority for purposes counter to their preferences.

Such turf battles were a major factor in shaping Dodd-Frank and the following rule makings, as recounted in the memoir of Sheila Bair, a former chairwoman of the FDIC who was appointed by President George W. Bush and continued serving through the crisis and drafting of Dodd-Frank under President Obama. Bair wrote critically of some of her fellow regulators in Bull by the Horns, suggesting that on some occasions they looked out for their own career goals or the needs of businesses instead of the public interest.

Whether the patchwork of regulatory agencies is an advantage or a disadvantage, one thing is clear: It's hard to consolidate or phase out a regulator once it's in place. Dodd-Frank did eliminate one agency perceived as particularly captured by business, the Office of Thrift Supervision, folding its responsibilities into the OCC. It also created another major regulator, however: the Consumer Financial Protection Bureau, charged with oversight of consumer finance. The agencies will likely remain constituted the way they are until the next financial crisis shakes things up.