[Dane]

Hello to all.
I study Ashley Book of Knots. In the beginning of Chapter 6, I found a rhyme:

Knots aft on the port si’
Forward on the starboard;
Opposite the left eye,
All around and inboard.

The Rigging Lanyard Knot
(Sailors’ Work Rhyme)

Clue me in, please, what does it mean the word si’ in the first line?
Thank you.