Since the middle of the 18th century "window" has meant the double-hung window. Its name comes from its two panels of glass, an outside panel that slides down and an inside panel that slides up.

Two hundred years have produced a number of changes. The size of the window, the number of pieces of glass in a panel, and the relationship of the window to the rest of the facade have changed with various architectural styles. In the 19th century manufacturers developed a system of counterweights and pulleys to hold the panels open without pins or sticks.

Aluminium has replaced wood as the framing material. With all of these changes, the double-hung window is still the one that most old house owners are likely to find.

Original windows should be retained whenever possible since their size and shape play an important part in shaping the character of a house. Unfortunately, wood frame windows using only a single thickness are not very energy efficient. Loose caulking, missing putty, and old weatherstripping reduce their ability to keep in warm air even more. Proper maintenance can minimize these problems.

Use the warm summer months to examine the condition of the windows. Check the way the panels fit the frame and each other. The bottom panel should fit tightly against the sill and the upper panel against the top of the frame. If you can see light coming through at any of these meeting places or feel any air movement, it will be a source of problems in the winter.

There are products that can help. Appropriate storms windows, not plain aluminium ones, will prevent most leakage of air but are expensive. Felt fabric weather stripping or foam strip insulation with a self-adhesive back can be tacked to the inside of the frame to fill in gaps.

Metal weather stripping is also available. It can be used at either the top or bottom of the frame. It is especially good for the area where the two panels meet.

There is a new product on the market that requires more time and effort to install than weather stripping but offers an alternative to people who want to keep their double-hung windows and not all storm windows or new windows.

The Quaker City Manufacturing Co. is now producing a replacement window channel, the part of the frame that hold the two panels. The old panels are removed from the frame along with the ropes and counterweights. The panel are slipped into a new channel and that is nailed to the frame.

The new channel eliminates the need for the old rope-and-weight system and their eventual repair. Available from most hardware store and lumber yards, the channel fit common window sizes.