Or, from http://www.cornwalls.co.uk/history/sites/trethevy_quoit.htm :
The chap's argument that the front chamber is the space beneath the backstone and the rear that below it, just does not wash. Firstly, there is no communication between the two spaces (see photos and VR panoramas on our website) so the sliding door access he describes is impossible, secondly, after the fall of the backstone the space above it would have either been filled with infalling cairn material, or would have been open to the elements (depending on whether Trevethy had a cairn or not, opinion is divided). In either case, it could not be considered a functional chamber.
The second bit from the Cornish Guide is just plain wrong. Cornish variant portal tombs have flanking stones either side of the "portal" stone, these are sometimes set so as to partially enclose an area in front of the portal stone. Trevethy is a well-preserved example of this arrangement, the Zennor Quoit is another, the Lanyon Quoit has been so badly disturbed that the original structure of the chamber is unknown, there are no traces of flanking stones or an antechamber.
For a good reasonably recent account of Cornish portal tombs, we recommend Barnatt's excellent "Prehistoric Cornwall - The Ceremonial Monuments" there are detailed plans of the chambers of all the tombs mentioned above on page 44.
Maggie & Keith