Using the futtock shrouds involves climbing the underside of an overhanging rope at about 45 degrees.
As well as climbing the futtock shrouds, most ships also allowed access to the top through the "lubber's hole" at the tip of the ordinary ratlines.
However, this was generally scorned by experienced sailors, and reserved for those on their first few trips aloft.
Any traditionally-rigged ship will have futtock shrouds, but on some modern ones they are strictly for structural purposes, with climbing forbidden.
These ships may also dispense with lubbers' holes, and instead opt for a "Jacob's ladder" that descends from the edge of the top to the ratlines vertically, rather than overhanging like the futtock shrouds.