when the area to be spanned exceeds that of existing systems then a new system has to be developed. The box girder and the truss bridge evolved together but at different paces depending on the technology available and the geographical location e.g. Europe or USA. Such was the case with the box girder as it evolved from the principle of the initial bridge beam but with the centre hollowed out. Its initial development was led by Robert Stephenson with the box girder so large that the rail went through it. The Britannia Bridge, spanning the Menai Strait joining Anglesey island to mainland Wales, was one of the earliest box girder bridges.