# Constraints and restraints

A constraint removes a parameter. The tied one is no longer free, and its value
follows its sources exactly. {ref}`constraining-parameters` is the
reference for the three verbs (`Refinement.tie`, `Refinement.tie_equal` and
`Refinement.untie`), the affine form they share, the fnmatch globs they take,
the four refusals each can raise, and how a tied row reads in the parameter
listing.

A restraint keeps a parameter. It adds a row to the residual pulling a bond
length or an angle towards a target, and the fit is free to disagree with it at
a cost. {ref}`restraining-a-distance` is that reference. Use a constraint for a
relation you are sure of and a restraint for one you are only mostly sure of.

A constraint does not have to name a model parameter as its master.
{ref}`named-variables` is the reference for `Refinement.add_variable` and
`Refinement.remove_variable`, which declare a quantity of your own for other
parameters to follow, and for the multi-source form of `Refinement.tie` that
lets one such variable be written in terms of others.

The parameter paths both take, and the one grammatical trap in them, are in
[](model.md). The symmetry constraints you never declare (a cell edge following
another, a coordinate confined to its site-symmetry direction) are created for
you and read the same way, with `TieSpec.user` false.