The tower grain was the finest unit of the British tower system of mass/weight, intended to represent the weight of a grain of wheat. It had the value of 45/64 of a troy grain (0.703125 troy grain) by definition, or 1/7680 of a tower pound. (Thus, reversing the definition, 1.42222222 tower grains = 64/45 tower grains = 1 troy grain.)
The history of the grain can be traced back to a royal decree in 13th century England:
- By consent of the whole Realm the King's Measure was made, so that an English Penny, which is called the Sterling, round without clipping, shall weigh Thirty-two Grains of Wheat dry in the midst of the Ear; Twenty-pence make an Ounce; and Twelve Ounces make a Pound. — Henry III of England
This text has been taken to refer to the troy pound, so that the reference to sterling pennies is purely symbolic. More correctly, however, the pound in question is the Tower pound, and it talks about the actual mass of real sterling pennies. The Tower pound, abolished in 1527, consisted of 12 ounces like the troy pound, but was 1/16 lighter.
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|
- ↑ Connor, R.D.; Simpson, A.D.C. (c2004). Weights and Measures in Scotland. East Linton.