When creating URLs, it's important to avoid special characters that have specific meanings or reserved purposes within the URL structure. Here are some special characters that are generally avoided or should be encoded in URLs:

Space: Use %20 or + to represent spaces in URLs.

&: Use %26 to represent the ampersand character.

?: Use %3F to represent the question mark character.

=: Use %3D to represent the equal sign character.

#: Use %23 to represent the hash or pound sign character.

**/ and **: These are used as delimiters in URLs, so avoid them or use them carefully depending on the context.

@: Used for authentication in URLs, so it should be used with caution.

$, +, ,, ;, =, ?, :, |, [, ]: These characters can have specific meanings in certain contexts and should be used cautiously or encoded if necessary.

%: As mentioned earlier, % is used for URL encoding, so if you need to include a literal % in your URL, you should encode it as %25.

< and >: These characters are used for HTML tags and can cause parsing issues if included directly in a URL. It's better to avoid them or encode them as %3C (for <) and %3E (for >).

**** (backtick): Although it's not commonly used in URLs, if included, it should be encoded as %60`.

" (quotation mark) and ' (apostrophe): These characters can be misinterpreted and cause issues, so it's recommended to avoid them or encode them as %22 (for "), and %27 (for ').

{ and }: These are used for special constructs in some URL formats and should be encoded as %7B (for {) and %7D (for }).

| (pipe): It's used as a separator in some contexts and should be encoded as %7C.

^, ~, [, ]: These characters are used in certain contexts (such as regular expressions) and should be used cautiously or encoded if needed.

****: This is used as an escape character in many programming languages and systems, so it's best to avoid using it directly in URLs.

Overall, when constructing URLs, it's important to consider the context and potential interactions with different systems and parsers. Encoding special characters using percent-encoding (also known as URL encoding) ensures that they are correctly interpreted by web browsers and servers.

It's generally recommended to stick to alphanumeric characters (a-z, A-Z, 0-9) along with - and _ for creating SEO-friendly and easily readable URLs. When in doubt, it's best to URL encode special characters to ensure compatibility and avoid conflicts with the URL structure.