The cFP-RLY-425 is a double-wide module. However, looking at the picture in the manual, the definition of double-wide is that it takes up both the module slot and the I/O connector slot. For example, Figure 2 in the cFP-RLY-425 manual (http://www.ni.com/pdf/manuals/373879b.pdf) shows a 4 slot backplane with 3 other I/O modules, 3 connector blocks and one cFP-RLY-425. Please note that the power consumption of the cFP-RLY-425 is a 1.45 Watts (page 1 of the manual). This restricts you to no more than 6 cFP-RLY-425 modules on a single backplane. However, this would not leave power for other I/O modules to be in the last two slots. Controllers can supply a maximum of 9 Watts to all connected I/O modules, which is an average of 1.125 Watts per module.