Philodendron species with posterior lobes have remarkable variation in the shape of the sinus, ranging from merely arcuate, such as in P. microstictum (Fig. 289), or arcuate with decurrent petioles as in P. morii (Fig. 292) to V-shaped in P. glanduliferum (Fig. 200), P. lentii (Fig. 263), P. squamipetiolatum (Fig. 386) and P. tenue (Fig. 406) to oblong in P. dwyeri (Fig. 155) to spathulate in P. basii (Fig. 76), P. hebetatum (Fig. 218), P. smithii (Fig. 372) and P. rothschuhianum (Fig. 339), to parabolic and P. sulcicaule to hippocrepiform in P. hebetatum (Fig. 225), P. lazorii, P. panamense (Fig. 297) and P. squamicaule (Fig. 374) or sometimes closed on live plants as in P. fortunense (Fig. 182), P. gigas (Fig. 194), P. pterotum (Fig. 311) and P. schottianum (Fig. 366). In the case of P. fortunense and P. subincisum (Fig. 401) the sinus may be closed even on pressed plants. The shape of the sinus varies greatly between live and flattened dried plants since the shape of the sinus varies depending on the angle at which the posterior lobes are turned up in relation to the midrib. Many species have the posterior lobes turned upward at an angle to the midrib on live plants (see for example 104, 130, 159, 186, 366, & 374) causing the inner margins of the posterior lobes to become closer to each other and thus decreasing the size of the sinus. For example when the angle of the posterior lobes is extreme, the posterior lobes might be closed but when the same blade is flattened and dried the sinus might become spathulate or hippocrepiform. The description of the sinus as presented in this work for Central American P. subg. Philodendron is exclusively that of the flattened sinus unless otherwise stated.
The sinus shape may be infraspecifically variable as in P. sousae, for example, with the sinus ranging from spathulate (Fig. 379) to parabolic (Fig. 380). This is in part related to the age of the plant with older plants bearing larger blades which have larger, more well developed posterior lobes.