Erythro and Threo Stereoisomers

This video shows how to recognize erythro and threo stereoisomers such as those you would get after a bromination of an alkene. I work through translating a perspective diagram with two chiral centers into a Fischer projection and checking your work by identifying the R and S designation of each.
(As a side note: DESPITE what some best selling textbooks claim, you CANNOT reliably predict stereochemical outcome of an alkene addition beyond trivially simple examples. I DO NOT recommend wasting time learning rules like "cis-syn-erythro" which are useless for additions to E/Z type alkenes which cannot be named with cis-trans and are useless if two different groups are added (ie halohydrins) To predict stereochemistry of alkene reactions, you MUST be able to visually anti and syn additions either in your mind or with a model)