The basic unit of weight in the British system is the grain - originally based on the weight of a grain of barley (but note that money was based on the grain of wheat - and that three grains of barley weigh the same as four of wheat). This grain is the troy grain - there is no other weight of the same name.
The weight of one grain is constant throughout the many different systems of British weights. As you will see below, the ounce and pound are anything but contstant, but have altered to meet circumstances over a period of over a thousand years.
The avoirdupois pound is the pound in general use today. As its name implies, it was intended to be used for weighing heavy goods. This pound is of 7000 grains, and is split into 16 ounces (each, therefore of 437.5 grains). Each ounce is divided into 16 drams (which my calculator makes of 27.34375 grains each - much more fun than metric isn't it?).
|16 drams||= 1 ounce|
|16 ounces||= 1 pound|
|7 pounds||= 1 clove|
|14 pounds||= 1 stone|
|28 pounds||= 1 tod|
|112 pounds||= 1 hundredweight|
|364 pounds||= 1 sack|
|2240 pounds||= 1 ton|
|2 stones||= 1 quarter|
|4 quarters||= 1 hundredweight|
|20 hundredweight||= 1 ton|
The Troy pound was of 5760 grains, and was divided into 12 ounces, so a troy pound is lighter than an avoirdupois pound, but a troy ounce (at 480 grains) weighs more than an avoirdupois ounce. The troy pound was declared illegal in 1878, but the troy ounce continues in use today for weighing gold. The troy ounce is split into 480 grains, and you will see 1/2 ounce weights marked both '240 grains' and '0.5oz'. However, the apothecaries system also has an ounce weighing 480 grains, being divided into 8 drams (sometimes spelled drachms) of 60 grains, each dram being split into 3 scruples, of 20 grains. To make things more fun, a 2 dram weight would be marked '3ij' - I think that '3' means 'scruples' (there being 3 to the dram), and the 'ij' being an old-fashioned way of quoting the Roman numeral 'ii'. It doesn't end there - there are 20 penny-weights to the troy ounce, so the 1/2 ounce weight mentioned above could also be marked as '3iv' or '10dwt'.
|1 ounce||= 480 grains|
|1 ounce||= 24 scruples|
|1 ounce||= 20 pennyweights|
|1 ounce||= 8 drams|
The wool pound was of 6992 grains, and was (of course) used for weighing wool. The clove, stone and tod mentioned above were also used.
The tower pound was used for weighing coins, and was of 5400 grains. I believe the name tower comes from Tower Hill, the site of the royal mint. This number of grains comes from the traditional weight of an English silver penny of 22½ grains (Troy, or grains of barley - the same as 30 grains of wheat), and 240 pennies to the pound. The tower pound was abolished in 1527.
The London pound, or libra mercatoria (trade pound) was 7200 grains (i.e. 15 troy ounces). This died out around the middle of the 14th century. One London stone was of 12½ London pounds.