Shopping for a lampshade can be frustrating if you don'know the proper size shade for your lamp base.

To choose the correct height and width for your shade you should first measure the lamp base from the bottom of the base to the top.  The top of the base is where the bottom of the electrical socket is attached to your lamp base.  You then measure the height of the Harp.  That's the device, usually made of metal, which the shade sits atop with a finial to secure the top of the shade to the Harp ).  The Harp is shaped like a harp and it attaches to the device on each side of your lamp socket. 

Measure for the lamp shade's bottom width by taking the measurement from the bottom of the lamp base to the bottom of the socket's base.  The shade you choose should have a bottom diameter no less than 2 inches the height of the lamp base and no more than 2 inches than the height of the lamp base.  For instance, if your lamp base is 20 inches tall, the bottom width of your shade should be no less than 18 inches wide nor more than 22 inches wide.

The top of your shade should be close to one-half the width of the bottom of the shade.  Tall vintage lamps that previously had a cylindrical shade will usually look better with the same cylindrical shape of lamp shade, so your bottom and top widths will be the same.  The Chinese urn type lamp looks better with the Pagoda type shade on the inverted V, as in /\ shape.

The correct Height of your shade should not exceed the height of the Harp, plus or minus 1/2-inch.  Measure the height of the Harp from the base of the lamp socket (where the Harp is attached) to the top of the Harp where your Lamp Shade will rest.  If you have a 12-inch Harp your shade should not be less than 11-1/2 inches nor more than 12-1/2 inches tall.

One other suggestion:  Be aware that the lamp shade buyer can change the size (height) of the Harp already on an existing lamp if finding the right size of lamp shade is difficult .  The better lamp shops sell Harps of all sizes.  Some old-fashioned Hardware stores continue to sell them. You can sometimes get lucky and find them online and on eBay.  Forget discount stores, as you'll just waste your time and gasoline looking.   (Don't you all just love spending a Saturday afternoon strolling the aisles of old-fashioned hardware stores?  The treasures one finds there!)  Harps usually sell for $2.49 to $4.99 in good lamp shops.  You might want to take your old Harp with you when shopping for a new one.

As a precaution for safety reason, make sure you can insert three fingers between the top of your light bulb and the top of the Harp.  You will need that much room to insert, attach or remove a light bulb.  Caution: Lamp Shade distributors usually caution the buyer to not allow the lamp bulb to be closer than one-inch to the lamp's shade, but I wouldn't have my lampshade be any closer to the light bulb than 3 or 4 inches -- to be on the safe side! 

Remember: Measure from the bottom of the lamp base up to the bottom of the socket in order to determine your lamp shade's bottom width and don't forget the 2-inch rule, plus or minus 2-inches! 

Remember: Measure the Harp from the bottom of the socket where it attaches to the top of the Harp and and don't forget the 1/2-inch rule  - plus or minus 1/1-inch.

Also remember: You can change the size of your lamp's Harp if you are having difficulty buying the right size Shade for you Lamp.

My Lamp for example:  When buying a Lamp Shade for a 20-inch tall think vintage lamp with a 12-inch Harp, I'm looking for shade sizes with the following type Tag:  top measurement/height measurement/bottom measurement.  My thin and tall (20-inch) Stiffel lamp requires an 18 top / 12 height / 18 bottom shade (18/12/18).