Also known aa a Bo'sun Chair.

I use both the Main halyard and the Jib Halyard attached to the ring
and double bowline around my chest & shoulders using a length of line
with the bitter end on the main with a Sliding Hitch, after I get above
the spreaders..

In my senior years I've gone to a Extension Ladder to climb the mast

http://community.webtv.net/tassail/ThomPage