In the old 70s song the woman sings: I’ve got a brand new pair of rollerskates you’ve got a brand new key.
What does one use a key for on a rollerskate?
The key would loosen the front and back parts. You’d adjust the skate, then use the key to tighten it again.
OK help out a non-skater. Adjust it how? Make the wheels closer to each other side to side? Front to back? How did this impact the skating?
There are two ends to a metal roller skate. One fits over your toe. The other fits around your heel. They slide together like two rails fitting one over the other. Link.
ETA: The adjustment helps it fit tightly on your foot.
We had all-metal roller skates that you strapped on to the bottoms of your shoes. The front and rear sections had a sliding attachment that allowed you to adjust the length of the skates. There was a leather strap/buckle that went around your ankle to hold the rear of the skate to your foot. There was a pair of ‘wing’ clamps that were adjusted with the skate key to hold the front of the skate to the sole of your shoe. Obviously, this worked best with hard-soled shoes, but those dominated our footwear at the time (late 50’s, early 60’s for me). Anyway, the key went over the end of a screw-gear that moved the clamps in and out, to tighten the front attachment.
The key was to adjust the length of the skate. Skates were one size fits all so you had to adjust them to fit. There were also clamps over the toe that had to be tightened so the skate wouldn’t fall off. The skates were worn with your shoes.
“Shoe-skates” had rollers attached to a boot, and you bought those to fit. They cost more and only serious skaters had them. You could rent them at skating rinks, like you do with bowling shoes at bowling alleyes.
I can’t remember if shoe-skates required a key for anything.
Now I have Happy Organ playing in my head. Thanks a lot. 
Looks like us oldtimers got us a simul-post!
I’d say you’ve done all right, for a girl.
Hey! MY post included a link to a photo !
(Don’t think I’m going to ask you to skate during Girls’ Choice now, 3acres! Even if you do have a truck!) 
Oops! I’ve totally blown it. No point in looking for my skates now.
I had the kind of skates that go on over your shoes, but I don’t remember a key. We just tightened 'em back up with the wingnut. 
But it’s the girl has the skates and the boy has the key. Why does he need a key, when he hasn’t got any skates of his own?
Have you ever noticed the stalker-ish nature of the song? He is avoiding her and is in his room with someone else when she comes calling. I think that “new key” means that the boy’s family has had all their locks changed.
Sigh. So little imagination out there. Anyway, maybe he lost his skates but held on to the key in case he found them again, or maybe he realizes it’s a great way to hook up with a keyless roller-skating girl.
No, you’ve got to read between the lines. The can’t-say-it-on-the-radio lyrics are “I’ve got a (newly) functional female sexual organ and you’ve got a (newly) functional male sexual organ.”
You’re probably just not old enough to have run across the ones that required a key. The key was actually a wrench with a square hole that turned a square-tipped adjusting screw. It was shaped like a wingnut so as to be easily used with thumb and fingers. I imagine wingnuts (more accurately wingbolt, I bet) were an innovation to get around the problem of kids always misplacing their keys.
ETA: Roller skates needing keys were common in the 50’s and somewhat into the 60’s.
The skates in NinetyWt’s link would tighten with a key. You can see the key mount just forward and above the front wheels in the side view. Here I’ve marked a copy of that photo to point out the key mount.
Looks like us oldtimers got us a simul-post!
And one of 'em has Alzheimer’s.
Now it makes more sense. I have only seen the kind that were attached to a the shoe, I had not seen the ones that you attach to your shoes.
Those skates were good for ripping the sole off a shoe and incurring Mom’s wrath.
:: preens::
:dubious: Hey
Double hey!