Into the top of the pillar is set a brass fitting called a 'spider' incorporating three grooves 120° apart. The spider ensures that instruments can be automatically centred over the intersection of the three grooves when the feet of the tribrach of the instruments are placed in the grooves .
In the middle of the spider is a removable central plug.
In addition, a spider may occur as part of non-standard pillars, for example:
Occasionally, spiders can also be found set into other structures such as water towers, church towers and sea walls.
They were also used on Bilby Towers.
15 instances of non-pillar spiders are recorded in the OSGB36 list .
Of these, 7 have been destroyed:
- North Anston Water Tower
- Worksop Water Tower
- Cement Mixer
- East Cowes
- Sea Wall
- Four Ashes Wireless Mast East
2 are possibly missing:
3 are unknown and/or inaccessible:
3 are known to exist:
- History of the Retriangulation of Great Britain