Mary's dress waves.

[If you missed The Latest Discourse on this particular subject, start here and here.]

the screen door slams
mary’s dress waves

Seven words. Springsteen uses seven words to open one of his greatest songs on one of his greatest albums. Those two lines set the scene, they are intended to be the beginning of the day, the sunrise in the morning, the start of the song cycle, and they instantly bring you into the scene. We’re at a house that has a door and there’s a screen door. The door is in motion; doors either open or close, and here we infer that it has opened, because now Mary is outside of it. She could also be coming in, but the next line - “like a vision she dances across the porch” now informs us that she is outside, and there is a porch, and a radio is playing Roy Orbison’s “Only The Lonely.”

dum dum dum dum de do wah
ooh yea yeah yeah ya
oh oh oh ohhh wah
only the lonely

Mary’s dress is waving, because she is dancing. She is dancing, not twirling or tapping her toes. She is dancing across the porch, a physical action, an emotional reaction to the lyrics of the song or her delight in hearing it on the radio, or in response to weather sufficiently clement to be outside on your porch with your transistor radio -- or maybe the radio is in the house, in my mind it could be either, she’s come out onto the porch because she heard the car driven by Our Hero pull up in front of the house, and she wasn’t sure who it was so she didn’t turn off the radio, she just stepped outside; she’s barefoot, the dress is some kind of cotton sundress that you would wear around a house that was likely not air conditioned -- I’ve always assumed that it’s summer -- it’s not fancy but it’s comfortable, it’s not the kind of thing you’d wear to go on a date but you’d wear it on a normal day to hang around the house or meet up with your girlfriends or run errands.

wave verb
to float, play, or shake in an air current : move loosely to and fro : FLUTTER
flags waving in the breeze

sway noun
the action or an instance of swaying or of being swayed : an oscillating, fluctuating, or sweeping motion

Wave is a verb, it is an active word. Sway is a description of a thing that is happening. Mary is dancing, she is in motion, her dress is waving. She is not standing there listlessly on the porch like Blanche DuBois, she is so taken by a moment in “Only The Lonely” that she dances to it.

There goes my baby
There goes my heart
They’re gone forever
So far apart

[This lyric is also genius, simpler, more direct, but also perfect in that in 14 words you know exactly what has happened and how the narrator feels, down to the depth of his anguish.]

Bruce uses this particular song by Roy Orbison as a deliberate device -- as Orbison describes in the next verse/chorus -- remember this song isn’t even two and a half minutes long! -- because ultimately about being brave enough to try for love, or try for it again:

Maybe tomorrow
A new romance
No more sorrow
But that's the chance
You gotta take

Just like “Only The Lonely,” “Thunder Road” is a song about the aching depths of unrequited love and being terrified of expressing it, the anguished depths of trying and failing (or never trying!), the torment of failing and finding the strength to get back up and hold your hand out or grasp the hand being held out to you. Or climbing into the car with your Romeo and being willing to see what happens next.

This is why Mary’s dress waves and doesn’t sway. Mary has agency, Mary takes action, Mary decides to pursue the small flame flickering in her chest. Mary makes decisions, Mary dares to dream. Mary does not stand on the porch while the wind blows through her dress. Mary climbs in.