I agree with @ColeValleyGirl that:

The three classes of 'time period' tags I suggest we discuss are:

1. 'Named' time-periods e.g. world-war-1, world-war-2, us-civil-war
2. 'Century' tags e.g. 18th-century, 19th-century
3. 'Decade' tags e.g. 1840s, 1790s

but where I diverge is on the use of Century and Decade tags.

I think many questions on this site can benefit from 1 (sometimes 2) time period tag(s), and 1 (sometimes 2) geography tag(s) which leaves 3, 2 or 1 tags available for other purposes.

If a question is tagged with a named time period then I see no need for further time period tags on that question i.e. I think these named time periods are more important to have than centuries or decades.

I started on this site thinking that century tags were more important than decade tags, and that decade tags should not be used. However, I now, after seeing many more Q&As, have come to the conclusion that decade tags should be preferred to century tags, and so, if 1 or 2 decade tags are clearly applicable to a question then I would use them instead of a century tag. If a question covers three or more decades of a century then I would fall back to using the century tag instead.

My reason for this is that I think century tags are too coarse, and particularly so for recent centuries. A tag for every year would be too granular, while a tag for each decade seems to be about right.

For example, I may want to quickly review our questions on East Lothian in the 1820s and 1830s, which I can do by typing [east-lothian] [1820s] [1830s] into the search bar (which currently only finds one Q&A, but imagine a site 10 or 100 times our size).

On the other hand a search on [scotland] [19th-century] in the search bar brings back a far less useful set of Q&As for me.

I agree with @ColeValleyGirl that:

The three classes of 'time period' tags I suggest we discuss are:

1. 'Named' time-periods e.g. world-war-1, world-war-2, us-civil-war
2. 'Century' tags e.g. 18th-century, 19th-century
3. 'Decade' tags e.g. 1840s, 1790s

but where I diverge is on the use of Century and Decade tags.

I think many questions on this site can benefit from 1 (sometimes 2) time period tag(s), and 1 (sometimes 2) geography tag(s) which leaves 3, 2 or 1 tags available for other purposes.

If a question is tagged with a named time period then I see no need for further time period tags on that question i.e. I think these named time periods are more important to have than centuries or decades.

I started on this site thinking that century tags were more important than decade tags, and that decade tags should not be used. However, I now, after seeing many more Q&As, have come to the conclusion that decade tags should be preferred to century tags, and so, if 1 or 2 decade tags are clearly applicable to a question then I would use them instead of a century tag. If a question covers three or more decades of a century then I would fall back to using the century tag instead.

My reason for this is that I think century tags are too coarse, and particularly so for recent centuries. A tag for every year would be too granular, while a tag for each decade seems to be about right.

For example, I may want to quickly review our questions on East Lothian in the 1820s and 1830s, which I can do by typing [east-lothian] [1820s] [1830s] into the search bar (which currently only finds one Q&A, but imagine a site 10 or 100 times our size).

On the other hand a search on [scotland] [19th-century] in the search bar brings back a far less useful set of Q&As for me.