Re, Si

https://doi.org/10.1351/goldbook.R05308
A @S05981@ face of a trigonal atom is designated Re if the @L03518@ of the trigonal atom appear in a clockwise sense in order of @C01082@ when viewed from that side of the face. The opposite arrangement is termed Si.
See also:
enantiotopic
,
prochirality
Source:
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2216 [Terms] [Paper]