To serious lingerie aficionados and collectors, the term "step-in" or "step-ins" refers to the the more tailored bias-cut panties which sit at or above the waist and have wide legs. Like the pretty pair of pink rayon vintage step-in panties with a floral bouquet applique, shown here.
In First Steps In Dressmaking, a work published during this period:
The pantie type of undergarment is widely accepted both because it is comfortable to wear and because it fits smoothly.This would not only be true of flapper dress, but for the decades which followed in which fashion icons such as Garbo and The Great Kate wore those lovely trousers and pantsuits. Just imagine either of those ladies wearing these vintage peach silk and lace underthings!
In the vintage sewing book's illustration, Fig. II, note the high waist and side button closure. (Scanned vintage book pages found here.)
In the text, you'll also see just where measurements ought to be taken to insure the best fit of a snug waist, "easy hip," and loose fit of the leg "to the desired point above the knee."
The book also recommends the typical fabrics of the time: rayon, silk and fine cottons; both plain and printed.
Ads at the time often refer to step-ins (and other lingerie) as being made of glove silk, which is a weave of rayon, silk, and, later, synthetics.
Many collectors believe these panties were specific to the 1920s and 1930s, however, ads for step-in panties appear well into the 1970s. Such later step-ins were made of the most common fabrics of the time, including nylon, but the step-ins appear less and less often in the illustrations as the decades pass...
Because step-panties were so comfortable and flexible, they were often worn for dancing -- including by professional dancers and earned themselves the name of tap pants or tap panties, after the popular dance of the day, tap dancing.
While there is some debate about whether or not the original vintage tap pants were less tailored than step-ins, modern tap pants (which were wildly popular in the 1980s) are certainly far less tailored than the original step-in. Even this flirty black lace and sheer nylon pair from the 1940s is less tailored -- and with a much shorter leg, adorned in so much ruffled sheer chiffon yet, likely not-so-good under pants and many other fashions. (Unless you don't mind visible panty lines. ...I might risk VPL for such incredible vintage panties!)
If you're going for fit and fashion "flare," you won't want to miss out on step-ins, so step-up and look for them everywhere. And under multiple terms. When shopping (searching online or off), be sure to widen your search to include tap pants, step-in pants, step-ins, etc. and don't limit yourself too tightly by decade -- some sellers may not be aware of all the options, terms, dates, and fabrics.