When Maxwell first introduced the concept of a coherent system, he identified three quantities that could be used as base units: mass, length, and time.
Another three base units (for temperature, amount of substance, and luminous intensity) were added later.
Since the acceleration due to gravity is local and varies by location and altitude on the Earth, weight is unsuitable for precision measurements of a property of a body, and this makes a unit of weight unsuitable as a base unit.
[2]: 156, 158–159, 165 [3]: 221 The range of defined prefixes pico- (10−12) to tera- (1012) was extended to quecto- (10−30) to quetta- (1030).
[citation needed] A few changes to notation conventions have also been made to alleviate lexicographic ambiguities.