Quote Originally Posted by M. A. C. Newsome View Post
It was a gradual thing, like all fashion changes.

The first real documentation I can cite for the use of the side, or knife pleat is when the Gordon Highland regiment began making their kilts this way in 1853. But that doesn't mean that this manner of pleating was invented in that year!

Nor does it mean that everyone began knife pleating their kilts in 1853, either. In fact, if you look at the famous portraits by Kenneth MacLeay, done in 1865-9, they all show box pleated kilts.

The Book of the Club of the True Highlander, published in 1880, states that the "newer" form of pleating (i.e. knife pleating) is incorrect, and that kilts should properly be box pleated.

And let's not forget that up until the amalgamation of the the Highland Regiments, some still wore box pleated kilts, the Seaforth being one prime example. Though doubtless by the time we get to the twentieth century knife pleating was by far the norm.
As Matt says, it was a gradual thing. Bob Martin is with out doubt the expert on the kilt and its development. Unfortunately he doesn't play on the Forum.

The important thing to realise is that this gradual change involved a change in the style of box pleats first from a balanced box to an unbalanced one, what Bob calls the box-knife, which generally required more cloth 5-6 yards. Although the balanced box pleat can still be seem in some Victorian pictures into the c1860s it probably disappeared in favour of the box-knife at around the same time as the knife pleat began to appear but there would always have been an overlap. This box-knife is what the Seaforths, Argylls etc continued to wear. I'm not sure if the RRS have gone for that style over a knife pleat.

Short answer to the OP? One has to differentiate between civilian and military dress. I'd say 50-60 years for civilian kilts i.e. by the late C19th most if not all civilian kilts were knife pleated. Some of the military kept it in line with Dress Regulations.