Analogue models are used to investigate the development of periodic instabilities during a wide rifting mode of continental extension. The results of modeling support that unstable lithospheric extension is accompanied by development of regularly spaced structures at two characteristics wavelengths: a crustal (shorter) wavelength and mantle (longer) wavelength. Both wavelengths are dependent on the brittle/ductile decoupling (i.e., dominance of the brittle rheology of the upper crust over the ductile behavior), as an increase in the brittle layer thickness and a decrease in the extension velocity (decreasing the ductile resistance) lead to an increase in both crustal and mantle wavelengths. The characteristic wavelengths also increase with increasing the amount of total extension, whereas variations in the thickness of the ductile lower crust have minor effects on the spacing of structures. These results are similar to previous analytical models and match well the wavelength of structures in regions undergoing wide rifting, as the Basin and Range, the Aegean, the West Antarctic Rift System and examples from the extensional systems of the China-Mongolia regions.