davidb wrote:Thanks Mike but you didn't answer the question.
Should the photographs be mounted on a PORTRAIT (i.e. 40w x 50h) or LANDSCAPE (i.e. 50w x 40h) orientated mount?
Sorry, I misread the question, although now I am not sure if I totally understand your meaning.
By convention, an image in landscape orientation would be shown in a 50W x 40H mount and a portrait orientation in 40W x 50H. Ultimately it's photographer's choice, although you would probably need a good reason to go against the norm if the end result was not to look strange. For example, I can visualise a small print in landscape format mounted vertically off centre in a portrait mount. Mounting off centre from the horizontal axis is not unusual and can help the overall appearance, especially if the difference is around 10%-15%, but the sides not being equal would be very unusual.