The red lines show block divisions which indicate that the blocks employed were fairly irregular in size. This was not a problem considering that most of the detailed carving work would have been done in place. Using blocks of any dimensions, fitting them into the space available, was a way to maximise the use to which the available material could be put. Similarly irregular block sizes can be seen on the near-contemporary Temple of Vespasian at the other end of the Forum Romanum.
The shaded areas on this drawing show the original Roman sections of the Arch of Titus.