Pronounced "loc," an initialism for Letter of Comment, the loc (or LoC) is the primary currency in fanzine publishing, the most usual form of the usual: what recipients send faneds in exchange for their zines. When written on a poctsarcd, it may be called a poc.
Loc is both a noun and a verb. Loccers and letterhacks do the loccing. The differentiation between the two is a matter of degree and quality.
Typically, locs comment on the content of the issue, but correspondents finding little to say about a particular issue may simply express thanks, impart news of their own or write complete non sequiturs. (Such letters are apt to be WAHFed instead of printed in the lettercol, but famously in the 1970s, Jessica Amanda Salmonson acknowledged receipt of many fanzines with letters giving explicit details about the medical process that transformed her from Amos Salmonson; unsurprisingly, many of those nonlocs saw print.)
A fanzine consisting mostly or entirely of locs is called a letterzine.
In 2007, Brad W. Foster published a one-shot named Locs. The 5 1/4- x 4 1/4-inch fanzine consisted entirely of illustrations of nonsense phrases beginning with the letters "LOCS" (e.g., "Larry Occasionally Creates…Something"), each by a different fanartist.