eruvin 38

some ropes are used to bind and restrict. to hold down the red heifer, to hold up the sotah's clothing.

and some ropes are used to measure, to allow freedom of movement.

(measuring the shabbat boundary should be done with a 50 cubit rope made of plant material. not iron chains, even though they would not stretch. other ropes are mentioned, each with their own uses. some ropes used to tie down and restrict, but the shabbat boundary is there is allow for movement and therefore there is room for leniency)

eruvin 38

making an eruv with one's feet

when shabbat and chag are adjacent are they 1 unit

or 2 separate units?