{open} answers PAD's stand-in for the character {. The stand-in travels through the rest of the rendering as plain text and is turned back into the brace when the page is written out, so the brace can never start a tag - not even when the text around it is handed to another tag or a pipe.

Together with {close} it writes a single brace pair in running text, such as the name of a tag in documentation. For more than a few braces - a script, a style block, a code sample - the {ignore} pair is simpler, and in an HTML page the entity { works too.

Not to be confused with the open= option of a data tag, which writes a text once before the first occurrence.