Interaction design laws
Weber's law
Also called Weber-Fechner law, Just noticeable difference
Whether people notice a change depends on its size relative to what was there before, so small changes to familiar things often go unnoticed.
Adding a gram to a letter is noticeable; adding a gram to a parcel isn't. The smallest change people notice is a fairly fixed fraction of the starting amount.
In products, small gradual changes are easier for people to absorb than one big redesign. The same rule explains why a price rise of a few pennies on a small item can feel bigger than pounds on a large one.
In practice
A news site that changes its layout a little at a time over months tends to meet less resistance than one that relaunches overnight.
Common mistake
Using it to hide changes people should know about, such as shrinking a product while keeping the price.
Where it comes from
Ernst Heinrich Weber described it in 1834 from experiments with weights, and Gustav Fechner built on it in Elements of Psychophysics (1860).