ST. LOUIS — The Parent Teacher Association is among the country's oldest child advocacy organizations and has become one of the most recognized groups in the nation.

But the PTA, which wrapped up its national convention in St. Louis earlier this month, has shrunk considerably since its heyday in the 1950s as more schools decided instead to have PTOs — parent teacher organizations.

From a high of 12 million in the 1950s, PTA membership has dropped to about 5.5 million today.

PTA? PTO? Although their names often are used interchangeably, there is a difference.

The PTA was founded in 1897 as the National Congress of Mothers and is based in Chicago.

It is a national organization with a lobbying arm in Washington, and it provides a network of services to its dues-paying members.

Schools with PTAs get the support of the national group — from training and sample bylaws to a brand-name lobbying voice in legislatures and Congress. Part of the dues paid at local schools goes to those state and national functions.

PTOs are independent local groups. The dues they collect stay at their schools. They are free to adopt any bylaws and rules they like.

PTA stalwarts call PTOs "PT Others" or "PT Only," in an effort to underscore the difference.

Both groups have the same function and goals locally: raise money to support school activities and engage parents in their children's educations.

Some say it's an ideological difference that is leading people to opt for PTOs. Parent groups want to focus on local issues, and keep the funds within the school community.

"If you survey local groups to see what they're about, it wouldn't be about lobbying Washington," said Tim Sullivan, publisher of PTO Today, a magazine that provides guidance to PTOs and is distributed to every school in the country. "It would be about making their school a special place."

Now the PTA is trying to make a comeback, stressing the value it provides.

"PTA develops and trains leaders, and provides resources for the continuation of developing those leaders," said Kathy Nevans, president of the Missouri state PTA.

"One of the things we've always said is PTOs stand for 'own,' whereas PTA is for 'all' kids," she added.

The national PTA this month installed its first male president-elect, a move that may draw attention and spark membership.