I noticed that the letter "z" in Spanish is pronunced /θ/ in all circumstances, and the letter "c" is prononounced /θ/ in front of "e", "i" and /k/ in front of other vowels.
My questions are:
What is the origin of these two distinct spellings? Did Latin have a more practical reason for having two different letters that represent the same sound ?
Why hasn't modern Spanish reformed this redundancy, just using "z" everywhere ? Most other aspects of Spanish orthography seem to be completely coherent and phonetic.
A good example is the verbs ending with -izar, such as tranquilizar. In the first person preterite singular, it becomes tranquilicé, whereas *tranquilizé would be pronounced the same.