To not hide the generated thumbnails in obscure directories and thus don't make them locateable for the user, the directory for storing the thumbnails is named .thumbnails.
In the context of this paper there will be two kind of .thumbnails directories distinguished. First there is the so called 'local thumbnail directory' which is in the same directory where the original image lies. The second one is the 'global thumbnail directory' which is located in the home dir of the user. Of course the global dir is also the locale one for the users home dir.
If an application searches for a thumbnail it should query the local thumbnail dir first and if there doesn't exist a scaled down version should look into the global one. If this lookup fails too it must create a new thumbnail and try to store it in the local dir. Due to the lack of write permissions this may fail so that the newly created thumb must be stored in the global dir.
This basic mechanism described so far will be refined in the upcoming chapters.
Each .thumbnails directory has the same structure. Since this standard supports different thumbnail sizes (see 'Thumbnail Size' section) there are multiple directories within it. Most of these follow the nameing scheme 'width'x'height', which means that all thumbnails stored in these directories are 'width' pixels wide and 'height' pixels tall. The 'fail' directory will be discussed in the Thumbnail Generation Failure section:
.thumbnails/
.thumbnails/48x48/
.thumbnails/64x64/
.thumbnails/96x96/
.thumbnails/128x128/
.thumbnails/160x160/
.thumbnails/192x192/
.thumbnails/fail/
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Note: The number of thumbnail sizes and the sizes themselves are preliminary. There need to be some more discussion about sensible values.