x1 (usually Lojban word) is the BAI cmavo corresponding to root word/gismu x2.
Example: "zo bai baicmvo zo bapli". In general, "fi'o [le se baicvmo] fe'u" must be grammatically equivalent to [le baicmvo". Other criteria for other Lojban-related languages apply as required. See also bai, cmavo.