The SCAYT option is only for those using the legacy editor. The modern (new) editor uses the built-in browser spell-check.
Spell check can be enabled by default under Settings > Basic. The spell checker tool in the editor is SCAYT, which stands for "spell check as you type".
The SCAYT feature allows you specify a variety of options, including:
- Ignore All-Caps Words
- Ignore Domain Names
- Ignore Words with Mixed Case
- Ignore Words with Numbers
You can also choose from the following languages:
- American English (default)
- Brazilian Portuguese
- British English
- Canadian English
- Canadian French
It also allows for a user generated dictionary that is store in a browser cookie. Once too large for a cookie, you can store the dictionary in your library.
Most modern browser have a built in spell checker, which is why the feature is off by default.