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I'm not understanding when to use either. I sometimes see a tal:block nested in a metal:block, and sometimes the either way, or sometimes just one of the two.

What's the difference between them?

A <metal:block> tag allows you to use METAL statements without having to prefix each attribute with 'metal:', while <tal:block> does the equivalent for TAL statements.

One uses these when there's no obvious or necessary tag in which to put a METAL or TAL declaration. It's quicker to write than a div/span plus a tal:omit-tag.

Note that the name of the tag after the namespace (block) can really be anything. But 'block' seems to be our de facto standard.

Could you point me to a document where the usage of tal:block is explained. I could not find it in the Zope Book or the Zope Developers Guide.

It's kind of not explained in anything official last I looked. Beats me as to why. The posts in this thread are about as good as you're going to get, and there were some past explainations on the list. Dunno if there's a Zope.org howto or a Zopelabs recipe. If not, there should be. --jcc