Measurement methods for placement of the F3, F4, P3, and P4 points differ. If measured front-to-back (Fp1-F3-C3-P3-O1 and Fp2-F4-C4-P4-O2 montages), they can be 25% "up" from the front and back points (Fp1, Fp2, O1, and O2).
Measurement procedures for pinpointing the F3, F4, P3, and P4 locations differ. When assessed from a head-to-tail orientation (e.g., Fp1-F3-C3-P3-O1 and Fp2-F4-C4-P4-O2 configurations), these points can be 25% "over" their respective front and back markers (Fp1, Fp2, O1, and O2).