Two quantities with dimensions D1 and D2 can only be added or subtracted if (and only if) D1≡D2. For example, it does not make sense to add a length to a mass

Two quantities with dimensions D1 and D2 can only be added or subtracted if (and only if) D1≡D2. For example, it does not make sense to add a length to a mass. That’s not true though. Some units cannot be added together, for example all intensive units like speed, density, temperature etc.