My first encounter with an airline lounge was in the early 1970s at the Philadelphia airport when I noticed an oak door with a small brass plaque that said “Admirals Club.” I figured it had something to do with the Navy.

Those discreet plaques certainly didn’t look like invitations to enter a club. “That big wooden door didn’t feel very welcoming,” said Nancy Knipp, who used to work in the American Airlines Admirals Club system and now is a senior vice president for growth strategy at Airport Lounge Development, a company that develops independent airport lounges in the United States.

Back then, after decades of operating as invitation-only clubs for airlines’ most-favored customers, airline lounges were only starting to offer annual memberships to anyone who cared to pay. Now, of course, the lounge is a familiar and sometimes even crucial aspect of business travel — to the point where many fliers gripe about overcrowding.

Airport Lounge Development is a leading player in operating alternatives to the airline lounge, at a time when loyalty to an individual airline is lessening. Around the world, lounges not directly affiliated with an airline are common, but only lately has the independent lounge concept begun making inroads in the United States. Airport Lounge Development recently opened its sixth lounge, The Club at PHX, at the Phoenix airport. Others are at the Dallas-Fort Worth, San Jose, Calif., and Atlanta airports, and there are two at Las Vegas.

“In the U.S. market in particular, the airline lounges have historically really been almost the only way to go, but what we call common-use lounges have been very popular in international markets,” said Knipp, whose company has plans to open several more of these clubs in domestic airports this year, with expansion planned beyond that.

“It’s an opportunity for people who maybe don’t have an airline affiliation, or don’t necessarily want to pay the price for an airline lounge,” she said. Basic annual memberships to airline clubs typically cost about $500, with some prices up to $750 if guests are included in the membership. Most airlines sell day passes to their lounges for $50.

Airport Lounge Development clubs are run on what the company calls a “pay as you go” model. A day pass is $35, and the lounge amenities match most higher-end airport lounges, with showers, free appetizers and drinks, cozy seating and business facilities.

As many business travelers know, the airport lounge offers not only sanctuary from the terminal hubbub, but also access to better airline customer service, which is especially valued during times when flights are delayed or canceled. The airline industry is forecasting growing demand this summer, when memories will still be fresh of this past winter’s near-record number of flight cancellations attributed to bad weather.

A competitor of Knipp’s company, Swissport’s Airspace Lounge, operates independent lounges at John F. Kennedy in New York, Baltimore-Washington, Cleveland and San Diego airports, with day-pass costs that vary, partly depending on time of day. I found a range of $20 to $45 for a day pass at the one at Kennedy, for example.

Airspace Lounges are also free to holders of American Express Platinum cards. American Express has also been developing its Centurion luxury lounges, now operating at the Las Vegas and Dallas-Fort Worth airports. The company has announced plans for three more Centurion clubs — one at La Guardia scheduled to open this summer, and others at San Francisco and Miami by the end of this year.

American Express began more aggressively promoting the Centurion lounges last year as its Platinum card holders expressed dissatisfaction when the company dropped free access to American Admirals clubs. Platinum card holders also get free access to Delta Sky Clubs, and to 600 airport lounges around the world, including more than 30 in the United States, that have affiliations with the Priority Pass lounge network.

As for Airport Lounge’s domestic clubs, I had a connection through the Dallas-Fort Worth airport over the weekend, with enough time to seek out a lounge. The Admirals Club lounge in Terminal C was spacious and welcoming for a $50 day fee, $15 more than the independent club. And when I asked around about the Airport Lounge club (which is in a different terminal), I got shrugs.

“Oh, maybe you mean that brand X club in Terminal D that Qantas uses?” a woman at the Admirals Club entrance said.

So maybe Airport Lounge Development has a branding challenge as it continues to expand in this country.