A unit of weight that's accepted all over the world. However, coin fans should bear in mind that not every ounce is the same. Several versions of the measure were used historically including the Apothecaries Ounce and the Tower Ounce – a unit that dates to Anglo-Saxon coinage and which was abolished in favour of the Troy Ounce by Henry VIII in 1527. Today, only two forms are in widespread use. The International Avoirdupois Ounce which is equivalent to 28.3 grams or 437.5 grains is the everyday measure most commonly found in retail, catering and recipes. The International Troy Ounce which is equivalent to 31.1 grams or 480 grains is of far more interest to collectors and investors. It's the internationally accepted standard for precious metals and, although many metric gold and silver coins are also available, the 1oz bullion coin remains the most popular buy.