the zigzag can be very narrow to give it the stretch. Another way is to use a double needle. I use this whenever the hem comes down in a t-shirt. the topside will look like a double row of stitches and underneath will look like zigzag because of the one bobbin thread - but it stretches!

They were wrong. Any ZZ attachment will work except the one meant for high shank and slant shank machines.

How would you use a double needle on a 15? I'd have thought a double needle would suicide on the first attempt, as there's only space for one needle.

I was thinking the same thing - needles flying into someone's eye as it hits the hole...

Schmetz makes a twin needle that only has 1 shaft. It fits the same as a single needle. I know there are other brands that also make them. I have not tried one yet in any of my vintage Singers, but it works fine in my vintage Bernina, but then it is a zig zag machine. There are also different sizes.....meaning the space between the two needles. Biggest issue of course is the single hole in the straight stitch plate, lol!
i would suggest to the OP that she purchase a Singer 401 or 500, etc. oh darn! HATE it when I have to buy a new "vintage" machine!, lol. I have (4) 401's and (2) 500's. Love, love the 401's! Worth every penny I've spent.
Or....for you more mechanic minded....would it be possible to "bore" a slightly bigger hole in the stitch plate to accommodate a twin needle?

You can get those one shaft needles on the machine but last time I looked a straight stitch needle plate has room in there for only one needle to pass through.

Yep, that's exactly what I said in above post......;) So I wondered.....is it possible to slightly modify the stitch plate.....bore it out a tad? I am sure some on this list know the answer to that, lol. Or would it make more sense to just purchase a vintage zig zag? I know some don't care for the 401's, but I personally think they are the next best thing to store bread, lol!

I think it would be more trouble than it's worth. After you enlarge the hole you would still have to replace the bobbin case with a zigzag 15 class case. I also suspect there may be other issues like clearance on the feed dogs and maybe even timing issues. I certainly haven't seen every twin needle or zigzag capable machine out there but I think the ones I own are either drop in bobbin or the bobbin faces toward you as well.
By the time you modified a straight stitch machine to take dual needles you would have far more time and money in it than the $10 it takes to buy a zigzag machine at your local thrift store - at least that's where the prices start here.
Rodney

Wow I had no idea. That's how naïve I am about that kind of thing. Thanks for clarifying.

I think I'm most likely wrong about the part about which way the bobbin faces but still, these days it's far easier to find a twin needle/zigzag capable machine than it is to modify a straight stitch to accept a twin needle.
Rodney