Smart dashes let you set up TextEdit to automatically convert double hyphens (--) into an em dash (—) as you type.
To apply smart dashes as you type:
Choose Edit > Substitutions, and then choose Smart Dashes from the submenu.
To automatically use smart dashes in all new documents, choose TextEdit > Preferences, click New Document, and select “Smart dashes” below Options.
To apply smart dashes to text you've already typed:
To use smart dashes for only part of a document, select that part, choose Edit > Substitutions > Show Substitutions, select Smart Dashes, and then click "Replace in Selection."
To apply smart dashes throughout the document, choose Edit > Substitutions > Show Substitutions, select Smart Dashes, and then click Replace All.
Smart dashes are automatically applied to any additional double hyphens you type in the document.
To stop using smart dashes:
Choose Edit > Substitutions, and then choose Smart Dashes from the submenu to deselect it.