this nomenclature is used only in those compounds which hav....

 1) only two chiral carbons

 2) the followin structure...... R'-Cab-Cbc-R" i.e. out of 6 substituents on 2 asymmetric carbons, atleast 2 should b same...

..... when 2 like groups in fischer projection formula are drawn n d same side of the vertical line, the isomer is called erythro form.....if these r placed n d opp sides then it is threo form...

its ok....no need 2 thank me....i js wantd 2 help...