After examination and assessment, sites have been awarded a grading of quality on a four-point scale, as explained in the accompanying summary. Many important sites contain or adjoin areas where intrinsic nature conservation value is lower than the rest, rating around grade 4 or less. Often, these lower- grade areas contain different ecosystems from the more important parts of the site. Their inclusion with the site thus gives a bonus, enhancing the diversity and overall conservation value of the whole. This additional interest is mentioned and often described, but otherwise only key sites (grades I and 2) are the concern of the present report. Some sites contain high- quality examples of more than one major formation; in such cases the different main components are graded and described separately under different formations, though they lie together within a single continuous area. All sites have been assessed regardless of their existing conservation status, i.e. whether or not they are NNRs or any other category of reserve; this part of the Review has been concerned expressly with application of uniform standards in site assessment and selection according to intrinsic merits (except for the criterion of recorded history).

Sites of international or national (Great Britain) importance, equivalent to NNR in nature conservation value (many are already declared as NNRs); these are shown in capitals in the site indexes in vol. 1. Internationally important sites are denoted by asterisks. The safeguarding of all grade i sites is considered essential if there is to be an adequate basis for nature conservation in Britain, in terms of a balanced representation of ecosystems, and inclusion of the most important examples of wildlife or habitat.

Sites of equivalent or only slightly inferior merit to those in grade 1. These are thus also of prime importance but many duplicate the essential features of related grade 1 sites, which should have priority in conservation. Many can, however, be regarded as alternatives to grade 1 sites should it prove impossible to safeguard these.

Grade 1 and 2 sites are the actual places identified as exemplifying the abstract concept of key areas.

Sites of high regional importance, rated as high-quality Sites of Special Scientific Interest, but not of NNR standard. Regional, as distinct from national, criteria apply; some sites in this grade would receive a lower rating if located in a region with more extensive natural or semi-natural ecosystems, or a higher rating under converse circumstances.

Sites of lower regional importance, still rated as SSSI. Some such sites would not qualify if they were located in a region with more extensive representation of the particular ecosystem.

Grade 3 and 4 sites contribute to the total national requirement for key areas for conservation. Grade 1-4 sites cover a small part of Britain and the safeguarding of all grades is important for nature conservation. In the much larger proportion of ungraded land and water only a relatively small extent is virtually devoid of nature conservation interest, and consists mainly of urban and industrial areas covered by buildings or other man-made surfaces. The remaining very large total area of natural, semi- natural and artificial ecosystems has a variable nature conservation interest but is tremendously important in the aggregate. No attempt has been made in the Review to work out further gradations below grade 4, because an extension of this scale is inappropriate for land which cannot be scheduled within any one category. In the present system, the whole of a site is covered by its grading, though the subsidiary features of some composite sites have sometimes been given a lower grading under their own formation, in order to emphasise consistency of standards in assessment. For unscheduled land, a differential grading on a smaller scale is usually needed, but there are considerable difficulties in working out a system which could be applied throughout the country Such a system of evaluation is nevertheless urgently needed.

The assessment of international importance of sites involves application of the same criteria discussed previously, but the background scale becomes expanded to include the rest of Europe or even the whole world. In particular, high international importance is accorded to habitats, communities and species which are rare on the global scale. In Britain, sites thus designated are mostly high-quality examples of ecosystems and communities, and contain species with a very local occurrence in Europe, and some represent an end-point to an ecological gradient of continental scale. The label ' international importance' given to some key sites is essentially subjective and arbitrary in most instances, and the category should be regarded as open to revision in the light of views from international bodies or experts with wide overseas experience. Only in the cases of wildfowl and waders have any quantitative yardsticks of assessment been followed, but in this review international status (grade 1*) is based on the total biological importance of a site.

It is obvious that within the list of grade i sites, there are varying shades of importance. The internationally important sites naturally take precedence on the whole, though any particular one is not necessarily more highly rated than any of those not given international status. The most outstandingly valuable sites are those which rate highly according to several major criteria, as in the case of the North Norfolk Coast. In general, sites which are large, have high diversity (especially including more than one formation) and contain large populations of interesting species (especially rarities), rate very highly. However, the question of comparative importance between sites of disparate character, even within the same formation, leads to enormous conceptual and practical difficulties, and is not pursued further here.