Find solutions for mounting switch plates and electrical devices no matter your screw hole spacing. Note that for measuring purposes, distances are measured from center to center. Screws for wall plates and devices have the same thread pitch - just the head of the screw and its length vary.
Cover screws directly into tabs on handy box. Typically used for blank, louvered, cable or other devices that use the plate itself as a mounting bracket.
Screw spacing is 3.25" apart (center to center).
Also know as interchangeable mounting. Plate screws into device strap. Typically used on covers for Decora, Despard and other strap mounted items.
Screw holes are 3.812" from center to center.
Plate screws directly into holes on device. Common for toggle switches, single outlets, lamp holders, rotary dimmers, etc.
Device mount switch plates have screw holes 2-3/8" apart (center to center).
These blank aluminum covers are 6-3/8 tall x 4-1/2 wide. The two screw holes are spaced 4 inches apart.
Replacement covers for doorbells with 4.5 inch screw spread.
Device Mount without Device:
In this case, plate screws into a strap that has device mount holes.
(Straps are purchased separately.)
Box Mount Through Device:
Deep switch plates (like this rocker one) may have different screw hole placement than standard plates.
If the standard plate is strap mount, the holes must be moved inward to device mount spacing to accommodate the larger beveled edge.
In this case, you'll remove the screws from device, then use the included switch plate screws to attach the deep plate and device to the electrical box at the same time.
Mounting Modular Frames:
Modular frames are installed just like the strap mount units.
The frames come with mounting screws, which go through the inside oval openings (box mount at 3.281" or about 3-1/4"). The rocker plate then screws into the outer round screw holes (strap mount at 3.812" or about 3-3/4") the same way it would screw into the strap of a rocker switch or block outlet.
Custom Screw Spacing:
For certain applications, such as covering an old intercom, nurse pull station, or fire alarm box, combine triangular straps with box mounting straps to reach your box's spacing from 4.25" to 4.5". This will create a strap in which to mount a blank cover plate.
Then, cover with a blank plate that has standard box mount screw spacing.
Note that depending on the devices mounted in one electrical box, screw spacings will vary. Here is an example of a toggle/rocker combo plate with device mount and strap mount screw spacings.