The waymark that will not tell you which way to go
Sooner or later on a National Trail you pass a small acorn on a post. The National Trails site says the trails are signed with an acorn, sometimes with the name of the trail next to it, on stiles, gates and signposts, and that every National Trail in England and Wales uses the same one.
The National Archives holds the paperwork. A Long Distance Routes Committee was set up in 1954 under the National Parks Commission, and in January 1962 it noted that a single emblem would be worth having, used sparingly. Meeting notes mention the oak leaves and acorn the National Trust had used for some thirty years; by 1966 an acorn design was accepted in principle. Production began in 1968 at the Royal Label Factory in Stratford-upon-Avon. The Pennine Way carried it first.
It had to survive being carved into rock, fixed to a gatepost and printed on a notice, in metal alloy or plastic, four or six inches across, in two colours. The commission held that identity by shape counted for more than recognising what it depicted.
Not the direction: the waymark is non-directional, and the file gives it one job, confirming the line of the route where there is doubt. Not who may use the path either, which is the work of the coloured arrows. The Pennine Bridleway and the South Downs Way are bridleways, the Ridgeway is mostly bridleway and byway. It also stops at the border: Scotland has no acorn, its long routes are Scotland's Great Trails under NatureScot.
Sources: signage on the trails, The National Archives on the design, and National Trails on Wikipedia.