if the conjugation adds an 'a' or an 'o' to the stem, then the 'c' is changed to 'z'

mecer - to rock
mecerse - to rock back and forth
coercer - to restrict, to constrain
ejercer - to practice, to exercise
vencer - to beat, to defeat
zurcir - to darn (to sew up a sock hole is an example of darning)