This visual cue makes for an easy organization system, not much unlike the colored tabs you'll see on top of folders lined back into a file cabinet. (howtogeek.com)
We want to fold that line so that we can keep our leg facing forward with the direct line of force down to the ground. (weckmethod.com)
Alexander calder - hearts and spades: double page lithograph with fold line down the center. (chairish.com)