The 1940s throughout the 1940s (softening after 1945)
Shoulder Pads and the Square Shoulder
Padding sewn into jackets, coats, dresses, and even blouses to build a broad, squared shoulder line - the construction detail that carried the decade's whole silhouette.
Shoulder pads show up in three different eras on this site, and I think the 1940s version is the one that matters most. In the 1950s they're a softer holdover, and in the 1980s they're a power statement, but in the 1940s the padded, squared shoulder is the load-bearing wall of the entire silhouette. Take it away and the utility silhouette simply doesn't work.
The broad shoulder had arrived in the 1930s, pushed by Schiaparelli's tailoring in Paris and Adrian's costumes for Joan Crawford in Hollywood. The war made it universal. Military uniform put a strong shoulder line in front of everyone every day, and fabric restrictions made a padded shoulder the cheapest possible way to give a garment presence and proportion. Pads went into almost everything: tailored jackets and coats, obviously, but also day dresses, blouses, and knitwear. Many were sewn in with a few loose stitches, or even attached with snaps, so they could move between garments - a practical detail in a decade where you might own very few pieces.
Construction varied a lot with price. A good tailored jacket had a shaped pad built up from layers of cotton wadding or felt, graded thick at the shoulder point and feathering out toward the neck and chest so there was no visible ridge. Dress and blouse pads were simpler - often a covered half-moon of wadding - and home dressmakers routinely made their own from scraps, with pattern instructions for doing so. Getting the sleeve head to sit smoothly over a pad is genuinely fiddly, and it's still where I'd look first to judge the quality of a 1940s garment. After 1945, the shoulder starts to soften and slope, and by the time of the New Look in 1947 the ideal has reversed completely - which is why so many surviving 1940s jackets have had their pads quietly removed at some point.