I expect that the concepts of male preference primogeniture can be difficult to grasp in societies where ranks and titles are unknown.

The mechanics can indeed be confusing at first but so often they have had an effect upon history. It is rare that this practice is overruled unless it is by act or parliament under circumstances which are considered justified.

The Monarch creates a royal dukedom by letters patent and confers it upon a male member of the royal family (this is called a Creation). If that person then has male heirs the elder inherits the title unless he predeceases the next eldest brother and has died without male issue. If there are no entitled male descendants then that dukedom reverts to the Crown. Where a royal duke actually succeeds to the throne the title they held reverts to the Crown because even though they may have male heirs, the eldest Son would then qualify for the higher title of Prince of Wales. This title is not hereditary in the automatic sense and must be conferred by letters patent each time. Prince Charles, for example, was known only as Duke of Cornwall from his mother's accession in 1952 until when he was created Prince of Wales in 1958.

So as Bing says, Prince Edward is only fourth in line to the Duchy of Edinburgh. But because the first heir (Charles) would become the Monarch both the Dukedoms of Cornwall and Edinburgh would then revert. It would be more or less automatic that William would then be given both the titles of Prince of Wales and Duke of Cornwall but Edinburgh would then be up for a new creation by letters patent.

The "third son" argument put forward by MOR is different to previous practice. The last King that had so many male children was George III and his third son was created Duke of Clarence (a rare example of a third son becoming a Monarch - King William IV). It was Queen Victoria who created her second son Alfred as Duke of Edinburgh rather than Duke of York which was a break with previous tradition. Nor is the "second son as Duke of York" anything new nor anything to do with "pleasing the Queen Mother." Second sons of the Monarch have been Dukes of York for centuries with this one exception that I can think of.

To answer Bing's question from a royal title point of view I can think of one example. George III's fifth son Ernest Augustus was created Duke of Cumberland (last held by the one of Culloden). Upon the death of William IV he became King of Hanover because Victoria was disbarred by that kingdom's Salic law and he was then then eldest surviving legitimate male in the Hanoverian line.

The Duchy of Cumberland remained in his family even though it could be argued that it ceased to be a royal title after he (as the son of a British Monarch) and his son (as the grandson of one) had died. His grandson, the third Duke, was deprived of the title by act of parliament in 1917 for being, as a German, an enemy of Great Britain during World War I

http://en.wikipedia.org/wiki/Titles_...ation_Act_1917

Edward could have been given various royal dukedoms that are currently in abeyance such as Clarence or Sussex or Cambridge rather than being made Earl of Wessex so it would appear fairly clear that Edinburgh is earmarked for him once it becomes available.

With that in mind it is even more appropriate for him to be kilted when he visits the USA on this occasion!