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The system called tower weight was the more general name for King Offa's pound. This dates to 757 AD and was based on the silver penny. This in turn was struck over Arabic dirhams (2d). The pound was based on the weight of 120 Arabic silver dirhams, which have been found in Offa's Dyke. The same coin weight was used throughout the Hanseatic League.
The mercantile pound (1304) of 6750 troy grains, or 9600 tower grains, derives from this pound, as 25 shilling-weights or 15 tower ounces, for general commercial use. Multiple pounds based on the same ounce were quite common. In much of Europe, the apothecaries' and commercial pounds were different numbers of the same ounce.
The tower system was referenced to a standard prototype found in the Tower of London and ran concurrently with the avoirdupois and troy systems, until it fell out of use and was abolished in 1527.
Relationship between London and tower weightsEdit
|1 London pound||=||1⅓ tower pounds||=||7,200 troy grains|
|¾ London pound||=||1 tower pound||=||5,400 troy grains|
|1 London ounce||=||1 tower ounce||=||450 troy grains|
|1 London pennyweight||=||1 tower pennyweight||=||22½ troy grains|
Relationship between merchants' and tower weightsEdit
|1 mercantile pound (15 oz)||=||9,600 tower grains||=||6,750 troy grains|
|1 tower pound (12 oz)||=||7,680 tower grains||=||5,400 troy grains|
|1 tower ounce (20 dwt)||=||640 tower grains||=||450 troy grains|
|1 tower pennyweight (dwt)||=||32 tower grains||=||22½ troy grains|
Comparison of different English pounds
|Metric||1||≈ 15/14||≈ 11/10||≈ 8/7||≈ 4/3||≈ 10/7||≈ 16||≈ 171/7||≈ 173/5||≈ 7716||≈ 10974||500||1/2|
|London||≈ 14/15||1||36/35||16/15||5/4||4/3||15||16||1616/35||7200||10240||≈ 467||≈ 7/15|
|Avoirdupois||≈ 10/11||35/36||1||28/27||175/144||35/27||147/12||155/9||16||7000||99555/9||≈ 454||≈ 9/20|
|Merchant||≈ 7/8||15/16||27/28||1||75/64||5/4||141/16||15||153/7||6750||9600||≈ 437||≈ 7/16|
|Troy||≈ 3/4||4/5||144/175||64/75||1||16/15||12||124/5||1329/175||5760||8192||≈ 373||≈ 3/8|
|Tower||≈ 7/10||3/4||27/35||4/5||15/16||1||111/4||12||1212/35||5400||7680||≈ 350||≈ 7/20|