8" long tandem switch plates for covering two electrical devices stacked lengthwise. These specialty type long wall plates were common in apartment and condo developments, particularly in the Chicago area. Tandem plates were typically mounted sideways and configured with either a Decora receptacle or duplex outlet side by side with a Western Electric phone jack 548A or Sierra plug. Often, residents will remove the unused telephone jack and cover this long switch plate with the outlet opening stacked by a blank. Dimensions are 8.25" long x 2.75" tall, but the location of the devices within the plate may vary, so check spacing carefully.
For electrical boxes that are stacked one on top of the other, you can use short switch plates to cover your devices. Short switchplates are trimmed on one edge (the top or bottom) to reach the edge of the electrical box on 1 end, rather than extending beyond the edge of the box the way a standard size plate does.
Below, we have an example of two stacked electrical boxes.
You can see that both the toggle and duplex devices extend slightly beyond the electrical box's edges when installed. In order for the device tabs to not overlap, these electrical boxes are spaced 3/16" apart.
Note that because short plates only reach the edge of the handy box, a portion of the device tabs will be exposed between the 2 cover plates.
If the electrical boxes are spaced 1/4" part, you can use 1 short plate and 1 standard plate, which may cover the exposed device tabs.