thre-
combining form 
or threo-
Etymology: International Scientific Vocabulary, from threose 
1. : threose 
< threitol >
2. threo-, usually italic : having the same stereochemical arrangement of two asymmetric carbon atoms as that found in threose 
< threo-3-chloro-2-butanol >
or threo-
1.
< threitol >
2. threo-, usually italic
< threo-3-chloro-2-butanol >