1882 S. S. Hellyer iii. 108 About the first form of trap used for fixing under water-closets was the syphon or round-pipe trap, i.e., a pipe bent and recurved in the shape of the letter ?.]
1885 P. J. Davies I. 103 Fig. 205 is the ordinary half ?-trap, wrongly called ?-trap.
1976 R. Day 64/1 In a ground floor w.c. it is usual to fit an S trap,..but in an upstairs floor w.c., a P trap is usually installed.