The garment as first worn in the nineteenth century was a single one-piece union suit reaching to the ankles and the wrists, buttoned at the front, of the general shape of the underwear of the period. It has been altered several times since. Sleeves and legs were shortened in the 1920s under Heber J. Grant, a decision taken by the church leadership and communicated to members. A two-piece version, a top and bottoms, was introduced in 1979 and is now the ordinary form. Further revisions in recent years have produced lighter fabrics, additional cuts, and options intended for members living in very hot climates, alongside adjustments to the women's garment.
Two things follow from that history. The first is that the design is not itself held to be revealed: it is administrative, it is announced, and it changes when the leadership decides it should. The second is that what does not change is the covenant the garment refers to, which is why members describe alterations to the cloth as unremarkable and alterations to the practice as another matter entirely.
That distinction is worth stating in a reference work, because outside accounts frequently treat a change of fabric as though it were a change of doctrine. It is not, and the church has been consistent about which of the two it is doing.