1) Fractions of a second 2) Fraternity letters 3) Greek consonants 4) Greek letters 5) Letters from greece 6) Odysseus trio, to Homer 7) Sibilants for Socrates 8) Sorority letters 9) Summation signs 10) Symbols in calculus 11) They follow rhos Found on https://www.crosswordclues.com/clue/sigmas