Scientific units are:<p>1. Standardised.
2. Defined by a numerical relationship to observable physical constants, like c, <i>by definition</i>.<p>Desk widths aren&#x27;t a scientific unit. You can only measure the ratio of width to height by reference to a standard unit like the metre, which in turn is based on a constant observable quantity - the distance travelled by light in one second.<p>You can pick your derived units using any relationship to c - like the distance travelled in 3.2 seconds. But that&#x27;s still a derived unit, not a fundamental observable unit.<p>And ratios are dimensionless because they stay the same whatever derived unit system you choose, as long as it&#x27;s consistent.