I'm a big fan of using joomla tags to classify items - articles, maps, films, books, people, reviews - and also using them to hold references and citations. Categories (in Joomla terminology) are also useful, but they have key limitations (which are advantages in some situations). Eventually the number of tags you are using across components gets unwieldy for the standard TagField form element to make it easy to remember and find the tag you want, whether using nested or Ajax mode.

Joomla database functions for building a query include a useful one for adding to the where clause with with a different condition (AND | OR) to the default you are using. 

But it has a gotcha if you are building a complex query with several possible WHERE elements

Tags:
Joomla:
Php:

In improving one of my Joomla components I found myself disappearing down a rabbit hole of confusion on t'internet over colour spaces, and in particular the differences between RGB, HSV, and HSL (plus of course for old TV hands like me YUV or Y'CrCb)

Joomla 3 has some strange restriction on the height of a modal popup in a component view. 

My workaround is this: