If you wanted to buy 50 shares of AT&T last week, at $51 a share, you could have paid any one of the following commission fees: $63 at Merrill Lynch, $63 at Dean Witter Reynolds, $48 at Fidelity Brokerage Services, $45 at Charles Schwab and Co., or $30 at Quick and Reilly.

The three last-mentioned firms are discount brokerage chains, one of them much cheaper than the others. So you even have to price-compare the discounters in order to find the best commission rate. or larger transactions, the price differences among the brokerage houses are even greater: to buy 200 shares of AT&T, you'd have paid $181 at Merrill Lynch, $178 at Dean Witter Reynolds, $70 at Fidelity Brokerage Services, $88 at Charles Schwab or $80 at Quick and Reilly.

Discounters are located in most large cities and many smaller ones. You find them by word of mouth, or by watching for their advertisements in financial pages.

But the discount business is changing rapidly. As the above prices show, some firms now discount more than others.

Low-cost brokerage services started back in 1975, when the Securities and Exchange Commission deregulated the commission fees for buying and selling stock.

The original discounters did nothing more than execute transactions. There were no fancy offices, no tape rooms open to the public, and very few customer services. All they did was buy and sell stocks and bonds, at prices 30 to 70 percent below those at the traditional brokerage houses.

The larger your order, the greater the discount.

Some discounters -- Quick and Reilly is a good example -- are sticking to the no-frills philosophy. But others, like Charles Schwab, are offering more services, and a slightly higher price.

All the discounters accept orders for stocks and corporate bonds. But some now deal in municipal bonds, government securities, mutual funds, stock options, even commodity accounts. Some send out stock-market letters, or provide their customers with low-cost subscriptions to market letters prepared by others.

Most discounters now will let you leave your securities in a custodial account at the brokerage house -- a big convenience for frequent traders. Many firms also offer margin accounts. A few will automatically sweep the idle cash balances in your account into a high-interest money-market mutual fund.

But all the discounters are still sticking to two fundamental principles:

Their brokers are salaried order-takers, not commission salesmen. They do not give investment advice. Indeed, in many cases they don't know enough about the market to hold a competent opinion.

Also, discounters don't get involved in costly stock-market timing analysis or investment research.

You use a discount broker if you make your own investment decisions and don't need any stock-market advice. All you want from the firm is a low-cost transaction.

A discounter is also good for people who do not normally invest, but have to make a one-time transaction. For example, if you inherit some stock and want to sell, a discounter can do it cheaply.

Discounters, however, do charge minimum commissions in the area of $20 to $35. So if you have only a few shares to buy or sell, a discount house will probably not save you any money.

Because of the low pay and relative inexperience of the typical order-taker at a discount house, these firms may make more than the usual number of errors. No matter which kind of stockbroker you deal with, always keep a close eye on your account to make sure that transactions are being entered properly. he traditional brokerage houses are not suffering this competition lightly. They may cut their commissions to wealthier investors--the very ones who have the most to save by working through a discounter. Some stockbrokers butter up their best customers with gifts, theater tickets and even cash kickbacks from the broker's share of the commission.

Small investors, on the other hand, still pay the full commission rate. In return, they get broad-based investment advice, and continuing assistance in managing assets -- a service that part-time investors generally consider worth the price.