I before E except after C
"I before E, except after C" is a mnemonic rule of thumb for English spelling. If one is unsure whether a word is spelled with the digraph  ⟨ei⟩ or ⟨ie⟩, the rhyme suggests that the correct order is ⟨ie⟩ unless the preceding letter is ⟨c⟩, in which case it may be ⟨ei⟩. The rhyme is very well known; Edward Carney calls it "this supreme, and for many people solitary, spelling rule". However, the short form quoted above has many common exceptions; for example: 

⟨ie⟩ after ⟨c⟩: species, science, sufficient, society
⟨ei⟩ not preceded by ⟨c⟩: seize, vein, weird, heist, their, feisty, foreign, protein
The proportion of exceptions can be reduced by restricting application of the rule based on the sound represented by the spelling. Two common restrictions are:

excluding cases where the spelling represents the "long a" sound (the lexical sets of FACE  and perhaps SQUARE ).