Considered the most universal of all the good luck charms, the horseshoe was introduced to our western culture by the Greeks in the 4th Century, who believed it symbolised the crescent moon, which was regarded as a symbol of fertility.
There is a nice story about the Devil asking a blacksmith to shoe his single hoof. When the blacksmith recognised his customer, he carried out the job as painfully as possible until the Devil roared for mercy. He was released on condition that he would never enter a place where a horseshoe was displayed. The blacksmith was St Dunston, who became the Archbishop of Canterbury in AD 959.
Because the horseshoes is "U" shaped, to retain the good luck forever it is essential that the horseshoes be hung by the ribbons that are attached to its shoulders. The horseshoe should not be turned upside down or else all the good luck of the marriage may fall out.
It’s also apparently good luck to see a grey horse en route to the church — even more so if the bride travels in a carriage drawn by a grey horse — whilst the luckiest horseshoes are said to come from the hind feet of a grey mare.