Most don't realize that during the Napoleonic time period (1793-1815), the coat worn by the British infantryman went through quite an evolution. At the start of the French Revolution, the standard issue coat worn by the British infantry looked something similar to the coat worn during the American War for Independence (1775-1783). I could try to describe the evolution, but I believe the following illustration does a much better job.
(Click for a larger version)
The illustration is from C. E. Franklin's book British Napoleonic Uniforms, which was just published. The following text is the caption for the illustration:
The evolution from the long tailcoat of 1793 to the final version of the jacket in 1815. The old pattern 'swallow tail' coat is taken from tailors drawings, as are all the illustrations. (1). The first style of coat introduced by the regulations of 1796 closed the front but retained the lapels (2). It should be noted that most regiments wore a button or some such ornament, at the point where the front and rear turn-backs joined unless they were secured by some other device. In the parlance of the day this was called a 'dubby or a dubbie'. The labels were removed in the same year (3) and the jacket shortened, but it took some time before the closed rear opening with a fly, known as a 'tommy', became a standard feature (4). The later version of the jacket was shorter and the final version had a higher collar to hide the clasp of the stock left uncovered by the loss of the queue and was shorter in the skirts with the horizontal pocket flaps stitched down to the front of the pocket and secured by two extra small buttons. The oblique pocket flaps were false with the pocket entered in the plait (5).
See British Napoleonic Uniforms by C. E. Franklin for more details.
Ich Dien!