Michael Ochs Archives via Getty Images A model poses walking off the Pan American Clipper "Challenge" Lockheed 1049 airliner circa 1947.

United Airlines blocked girls from wearing leggings on their flight Sunday, not only angering Twitter but also amplifying the difference in dress codes from when people first began flying.

No longer do passengers expect to have insane amounts of leg room, eat lobster as a main course in coach or see fellow passengers wearing red carpet attire, as they did starting in the mid-1930s.

When commercial flights first became popular in the ‘30s, the typical cost for a ticket could total up to 5 percent of one’s annual salary, making it a relatively rare and special event for the rich. Keith Lovegrove, author of Airline: Style At 30,000 Feet, told The Huffington Post that in the ‘30s, commercial flights didn’t have tiered classes because only rich people flew ― and of course, they dressed up for the occasion.

“Over the next few decades, as air travel became steadily more affordable for the middle classes, dress became less important,” Lovegrove said. “And as worldwide air travel is, as its most surreal, a parallel universe, trends and fashion mimic what happens on terra firma.”

Janet Bednarek, a history professor at the University of Dayton, told The Huffington Post that the major style shifts started in the 1970s.

“[Passengers] stopped dressing for airline travel about the same time they stopped dressing for other occasions, like shopping,” Bednarek said, noting that the Airline Deregulation Act of 1978, which meant the government could no longer control things like rates and routes, also occurred around the same time as the shift. “The more casual dress on flights is associated with deregulation but really that was coincidence, not causal. There was already a larger shift in the culture.”

Bednarek and Lovegrove both agree that the accessibility of flying and loss of its once-special feel contributed to the way passengers dress on planes now.

“Passengers dress for comfort rather than ‘being seen,’” Lovegrove said. “Budget air travel today is akin to getting on a bus, and we don’t dress up to get on a bus.”

Flight attendant Heather Poole told The Huffington Post that she doesn’t care what passengers wear, just as long as they’re nice.

“That said you can run a lot faster away from a burning aircraft if you’re not wearing flip flops,” Poole said via email. “Speaking of shoes, that’s not water on the bathroom floor, so you might want to keep them on.”

Good to know. Take a look at how attire has changed through the years in the slideshow below:

1927 Underwood Archives via Getty Images The official opening of an airline passenger service between Boston and New York took place when the first two women to make the night trip alighted from the air mail plane in New York on April 5, 1927. The fare was $25 per person. 1935 Hulton Archive via Getty Images Passengers sitting in their seats inside the cabin of a 21-passenger airplane making daily trips between New York City and Chicago circa 1935. 1939 Bettmann via Getty Images Passengers boarding Pan American clipper ship at Port Washington, NY circa 1939. 1940s Pictorial Parade via Getty Images Passengers, seated two to a row, talk with their inflight neighbors aboard a Mainliner Convair aircraft as the flight attendant works at the back of the plane in the late 1940s. 1949 Bettmann via Getty Images Stewardess Jane Driscoll lights a cigar for airline passenger Carl Graulein, 68, of East St. Louis, as he prepares to depart for his native Germany aboard an American Airlines overseas flight on Nov. 10, 1949. 1947 Michael Ochs Archives via Getty Images A model poses walking off the Pan American Clipper 'Challenge' Lockheed 1049 airliner circa 1947. 1955 Michael Ochs Archives via Getty Images Passengers enjoy a relaxing smoke on a Transocean Air lines Boeing 377 Stratocruiser circa 1955. 1960 H. Armstrong Roberts via Getty Images A family walking down airplane stairs. 1961 Francis Miller/The LIFE Picture Collection/Getty Images Passengers getting off of a tardy chartered plane on Oct. 1, 1961. 1974 New York Post Archives via Getty Images Miami-bound passengers on National Airlines go through a baggage check. 1977 Dick Loek via Getty Images Passengers taking a bus to get to a connecting flights in Niagara Falls, N.Y. 1980 Waring Abbott via Getty Images Passengers board an Aloha Airlines plane named after Hawaiian monarch 'Queen Liliuokalani' at Honolulu airport in Honolulu, Hawaii on Jan. 2, 1980. 1984 Jim Preston/The Denver Post/Getty Images Just for special snow days, United Airlines now has signs and lines to handle passengers with cancelled flights. They had four lines going and about six agents handling the frustrated travelers on Nov. 12, 1984. 1994 David Rentas/New York Post Archives/Getty Images Airplane passengers look through windows at airplanes being de-iced and plows at work at La Guardia airport on Feb. 11, 1994. 1995 David Butow/Corbis/Getty Images Shuttle by United travels most frequently between San Francisco and Los Angeles on Sep. 7, 1995. 2001 Tim Boyle via Getty Images Travelers stand in line at the United Airlines terminal at O''Hare International Airport in Chicago, Illinois on June 18, 2001. 2002 Chris Livingston via Getty Images Passengers wait in line at the low-cost carrier Southwest Airlines ticket counter at Orlando International Airport in Orlando, Florida on Dec. 13, 2002. 2004 David McNew via Getty Images Passengers arrive at Los Angeles International Airport (LAX) Nov. 23, 2004 in Los Angeles, California. 2007 Bill Pugliano via Getty Images Northwest airlines passengers check in at Detroit Metropolitan Airport May 31, 2007 in Romulus, Michigan. 2011 Joe Raedle via Getty Images Airline travellers stand in line to get to the United airlines ticketing counter at Newark Airport in Newark, New Jersey on Aug. 29, 2011. 2013 Sandy Huffaker/Crobis/Getty Images Passengers prepare to board their flight at O'Hare International Airport in Chicago, IL on Thursday, Oct. 10, 2013. 2013 Sandy Huffaker/Corbis/Getty Images Passengers aboard a US Airways flight take their baggage from the overhead bin as they depart the plane in Phoenix, AZ on Oct. 7, 2013. 2016 Victor J. Blue/Bloomberg/Getty Images Passengers walk through the Delta Airlines Inc. terminal at LaGuardia Airport in the Queens borough of New York, U.S. on Monday, Aug. 8, 2016.

Slideshow photos curated by Damon Dahlen