Can tags have hierarchy?

Does anyone have experience working with nested tags? I could not find any websites that allow users to have hierarchies in their tags (equivalent to non-mutually exclusive categories). Is it a bad practice in general? If so, why?
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What is User Experience (UX)?

I’ve looked at this site for the first time today. I’ve checked About, the FAQ and the FAQ and searched, and I don’t seem to be able to find an answer on this site.
These are the closest I can find:
https://ux.stackexchange.com/questions/7174/difference-between-ui-and-ux
What’s the difference between UX and layout design?
Reading the first few responses, I’m perceiving quite a difference in scope between ‘user experience’ as a concept and what I see discussed as ‘User Experience’ on this site and elsewhere, and I’m definitely interested in opinions addressing this.
Also, while I appreciate the field is broad, maybe there are some human interactions that definitely aren’t UX – between two of them face-to-face, for example?
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What is the ideal height of a button in regular desktop applications?

On most GUI creator (I just checked Qt Creator and Flex Builder), it seems the default button height is about 22 pixels. This is what I use for most desktop applications I create except those I make for myself. In this case, I use bigger b…
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Selection of multiple items and then selected one as default

I’m trying to design a web interface in which users pick particular “items” and set one of them as default. The textbox for searching uses autocomplete to find the matching item. I’ve come up with the following design to allow the user to …
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What is the difference between wireframing and prototyping?

What is the difference between wireframing and prototyping?
Am I right to think that prototypes should be interactive and wireframes should be static?
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International Currency Formatting Guidelines — Currency Codes

While developing a web application that presents monetary data for a variety of locales, I am inquiring about the differences of currency formats. In particular, I’d like to know when it is appropriate to use currency codes, either in addi…
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Progressively Filtering Options

I am modelling a selection from a product matrix. The customer envisions this as a series of dropdowns, which progressively filters what is available in a subsequent dropdown.
So for example, the first dropdown could be of "Vehicle Ty…
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Showing how many users voted that a comment was helpful

I’m building a site with items that can be reviewed, and users can vote whether or not reviews are helpful to bubble the best to the top Amazon-style.
Amazon, Yelp, and almost everything else I could find shows the number of people who fou…
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Best UI approach to align the column titles of reports

I would like to know what the best approach is to align header columns according to their data type.
I haven’t found any Human Design Interface articles explaining this.
Should the header column follow the same alignment that its data va…
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Rate vs Like/Dislike

I’m writing a web page that will allow people to rate “objects”. I was thinking of giving users the classical 5 star rating… but then I remembered the like/dislike approach and how YouTube changed from the 5 stars to the like/dislike met…