An advantage of the SI system decimal prefixes is that they make for simplicity of calculation and conversion involving units of different sizes; consider for example the simplicity of buying 13 items of 390 g weight at €12.34 per kilogram, compared with items of  oz at $4.79 per pound (or, worse, with old non-metric currency: £4/15/9½). In the units used in the US, combining of units that are not decimal multiples of each other is often avoided by not mixing the units used, e.g., using inches, feet or miles only: 89  inches rather than 7 feet 5 inches (or 2 yards, 1 foot 5 inches).