'Departure’ here is used as a noun premodifier to “lounge”, to be more precise, and the whole sequence functions as a single word both grammatically and semantically. It’s quite a fixed structure, and its semantic distinction as a single compound seems to be firmly institutionalized. The same goes for ‘departure gate/time. On the other hand, we have some compounds such as ‘departureS board’ or ’courseS committee’ where again, the degree to which they are institutionalized probably does the same ‘job’ for plurals.
edytowany przez savagerhino: 04 lut 2011