Restraints and constraints are often used in crystallography, but there is some confusion as to the verb and noun forms of these words. Namely, when using the terms as nouns, the “t” at the end of the word is necessary:

• “The restrains applied were…” WRONG
• “The restraints applied were…” RIGHT

However, the verb forms of the terms do not have an ending “t”:

• “The program does not constrain the data…”
• “We restrained the collected reflections…”

In which contexts does one use constraint vs. restraint ? Some scientists maintain that the two terms differ in meaning: namely, that a constraint  fixes a term in a refinement process to an absolute value, while a restraint maintains a term around a correct value via some sort of energy or weighting function (as used for example by AMBER). However, if you’re not (say) the developer of a refinement program, the terms can be used more or less interchangeably.