Interlocking charts use symbols to indicate the potential aspects of a lineside signal. But a layperson may not know how to translate those symbols to lineside signals.

In a nutshell, the symbols are semaphore representations of the position light signals. John Cooper provides this graphic to translate:

John Cooper also explains... "The signal symbol tells what arms are present in the signal. From there you can determine what bulbs are present. But from the arms, you cannot tell what aspects were possible. That is determined by how the circuits were configured. To know for sure what aspects were used, you need an aspect chart. These would tell you what aspects were displayed for every possible permutation of conditions."

"For pedestal signals, you can also determine which bulbs are necessary from the arms, but because bulbs are shared between arms, you can't tell what arms are present from the bulbs. Furthermore, on dwarf signals, the symbol does not tell you whether it was a left-hand or right-hand orientation - so you have to actually look at the fixture to determine which bulbs are there."

"Even without an aspect chart, you can get very close to figuring out what aspects a signal would display based on context."