At the latest from the 19th century on, the following two general principles have been adduced to explain linguistic variation and, especially, change at all levels:

• Principle of least effort: Maximum economy is a requirement facilitating use of linguistic units for the speaker.
• Principle of maximum distinctiveness: Maximum clarity is a requirement facilitating use of linguistic units for the hearer.

These two principlies are permanently antagonistic; in every individual case, one wins over the other. Adducing either of them to explain a particular phenomenon is therefore gratuitous. They may be useful for linguistic description, but are useless for explanation. (Horn 2008, CIL 18)