If doubtful about ie or ei, when the seem with the vowel is as Briefly /i:/, we spell it ie; but following the letter c, we spell it ei: Referer – prevalent misspelling of the word referrer. It is so popular that it made it to the Formal specification of https://arthurtqlhc.life3dblog.com/26310375/everything-about-spelling-rules-for-ssc-cgl