Trett was an allowance made up until the early 19th century, for waste, dust, and other impurity in items in commerce, generally amounting to 4 pounds (1.8 kg) in each 104 pounds (47.2 kg). It fell into disuse because merchants preferred to simply adjust the price, rather than make a calculation for trett.

Usage

Trett was most commonly used in calculating the true weight for imported commodities to Great Britain which would be sold by the pound avoirdupois

Avoirdupois

The avoirdupois system is a system of weights based on a pound of 16 ounces. It is the everyday system of weight used in the United States and is still widely used to varying degrees by many people in Canada, the United Kingdom, and some other former British colonies despite the official adoption...

. From the gross weight, tare weight

Tare weight

Tare , from the Middle French word tare "wastage in goods, deficiency, imperfection" , from Italian tara, from Arabic tarah, lit. "thing deducted or rejected," from taraha "to reject" weight, sometimes called unladen weight, is the weight of an empty vehicle or container...

 would be deducted, to account for the weight of any container that the commodity was enclosed in. The remainder was known as the suttle. Trett was deducted from the suttle at the rate of four pounds for every 104 pounds of commodity, to take account of any dust, sand, and other impurity included in the suttle. The remainder, after trett was deducted, became known as the "neat weight" or "nett weight", a term used in common for commodities in which no trett was to be deducted, but from which tare had been deducted.

Disuse

By the early 19th century, trett had fallen into disuse,
with merchants preferring to make allowances in the price for any impurity, as such allowance was far easier than the arithmetic calculation for trett. It was not used at custom houses, nor was it available at the British East India Company

British East India Company

The East India Company was an early English joint-stock company that was formed initially for pursuing trade with the East Indies, but that ended up trading mainly with the Indian subcontinent and China...

's warehouses.

In Ireland

Trett varied considerably from the British usage, for goods in Ireland. For most goods there, trett was deducted at the rate of one pound per 112. However, in Cork, trett was deducted after tare for wool at the rate of 8 pounds per 20 stone (280 pounds), and in Dublin 8 pounds per 21 stone (294 pounds). Additionally in Dublin, trett was deducted at the rate of 2 pounds per large cask of butter, 4 pounds per raw hide, and 8 pounds per beef carcasse.