Signs. That's the main subject for today.

* The National Park Service is usually very careful about the signs it erects. Yet its sign on the southwest corner of 17th Street and Constitution Avenue NW identifies the sturdy old sandstone lock-keeper's house as being part of the Chesapeake & Ohio Canal. Wrong. It's part of what was the Washington City Canal. (The C&O Canal ended at Rock Creek; the Washington City Canal, separately financed and operated, carried boats from the C&O Canal into downtown Washington via a waterway along what is now Constitution Avenue. It became virtually an open sewer and was filled in soon after the Civil War.)

* My colleague Nancy Scannell wrote of the inauguration of an Arlington thoroughfare -- Clarendon Boulevard -- in the Virginia Weekly last week. It's one step toward providing a continuous, and continuously named, eastbound twin to largely one-way-westbound Wilson Boulevard through the Rosslyn, Courthouse and (ultimately) Clarendon areas.

Alas, the two ends of Clarendon Boulevard are not yet connected, so, lacking directional signs, eastbound travelers must divine their way through the Courthouse complex. Not only that, but Arlington sign-erectors outdid themselves in one block of Rosslyn. A sign where Clarendon Boulevard merges into Wilson Boulevard at Oak Street labels a block of Wilson as being Clarendon Boulevard; at the other end of the same block, at Nash Street, it's correctly labeled Wilson Boulevard.

* Speaking of Wilson Boulevard, how come a real estate company was able to erect a permanent-type sign in the median of the 1100 block advertising offices for lease?

* And, in downtown Washington, how come a parking garage in the 1500 block of K Street NW has a big advertising sign on the dividing strip between K Street's traffic and service lanes? Historic Meeting

Washington lawyer and Arlington resident Hyman J. Cohen is a history buff (he's treasurer, among other things, of the U.S. Capitol Historical Society). For years, "a dream of mine," as he put it, has been to arrange a rap session among the four historical societies that operate in conjunction with the federal government.

On Nov. 5 he'll get his wish. The Capitol Historical Society, the Supreme Court Historical Society, the White House Historical Association and the Society for History in the Federal Government (the latter formed in 1979 as an omnibus organization for government agencies) will get together at the National Press Club.

Admission is free for the 6:30 p.m. public event (cash bar at 6 p.m.), but space limits dictate that reservations must be made. Call 662-7501. For information, call Cohen at 296-0486 (office) or 243-7633 (home).