Well, that's Trew I guess

[Dr Bee]

I've yet to find when and where this tradition arose.

The chances are that there may be no documentary evidence of how the tradition arose, but it's possible to make an educated guess. My guess would be that trews with no outer seem simply became the fashion for Army Officers.

The history of the introduction of tartan trousers, renamed as 'trews,' for the Scottish Lowland Regiments following the Cardwell reforms of 1881 is well documented and needs no rehearsal here. It is also well known that many Highland Battalions had worn tartan trousers as undress uniform for specific duties for decades prior to this. However, the enshrinement of tartan trews in the vestiarium of dress uniforms from 1881 probably led those who could afford it - officers - to purchase the best that could be had, perhaps initially for Mess Dress and also possibly for their Dress Uniforms. Seemless trews with the cavalry cut do look better and are certainly more expensive - they have the air of 'Officer' written all over them and they probably were made by the best tailors for officers who were required to purchase their own uniforms - a situation which persists to some degree to this day. Once such trews became accepted and approved of, they would come to be considered traditional for officers to wear, then essential and eventually mandatory.

My own experience as a Naval Officer has allowed me to glimpse one or two traditions in the wearing of uniform which will forever remain undocumented, e.g., the distinctive (to those who know) Gieves cap and badge was ordered to be worn for certain duties with the implication that failure to comply would earn eternal disgrace, ridicule or perhaps worse - but there were no written regulations to back up up such orders - it was simply a tradition - it had become 'the done thing.' Besides, who was going to argue against wearing the most comfortable officer's cap money could buy when the alternative was to wear the crown of thorns issued by Pusser. I know this is not an exact parallel, but I think it begins to illustrate how some uniform traditions which begin with officers buying the best they can get eventually become the accepted norm.