Halliday & Matthiessen (2014: 423):
Complex prepositions such as in front (of), for the sake (of), have evolved from prepositional phrases, with front, sake as ‘Complement’. Many expressions are indeterminate between the two … however, there is a difference; those that have become prepositions typically occur without a Deictic preceding the noun (in front of not in the front of), and the noun occurs in the singular only (in front of not in fronts of). In some instances duplex forms occur: beside has become a full preposition, but because it is often used in an abstract or metaphorical sense a modern version of the original complex form by the side of has reappeared along with it, and this, in its turn, is now starting to follow the same route towards prepositional status.