In the most traditional ships, the futtock shrouds can be used to gain access to the tops. Sailors ascend ratlines on the ordinary shrouds until nearly at the top, then transfer to the futtock shrouds which will be reaching upwards and outwards above them. Using the futtock shrouds involves climbing the underside of an overhanging rope at about 45 degrees. Futtock shrouds may or may not have ratlines.
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.