Try out our new RRULE tool for creating RRULE compatible strings.

Property Name

Any property name with a "X-" prefix


This class of property provides a framework for defining non-standard properties.

Value Type

The default value type is TEXT. The value type can be set to any value type.

Property Parameters

IANA, non-standard, and language property parameters can be specified on this property.


This property can be specified in any calendar component.


The MIME Calendaring and Scheduling Content Type provides a "standard mechanism for doing non-standard things". This extension support is provided for implementers to "push the envelope" on the existing version of the memo. Extension properties are specified by property and/or property parameter names that have the prefix text of "X-" (the two-character sequence: LATIN CAPITAL LETTER X character followed by the HYPHEN- MINUS character). It is recommended that vendors concatenate onto this sentinel another short prefix text to identify the vendor. This will facilitate readability of the extensions and minimize possible collision of names between different vendors. User agents that support this content type are expected to be able to parse the extension properties and property parameters but can ignore them.

At present, there is no registration authority for names of extension properties and property parameters. The value type for this property is TEXT. Optionally, the value type can be any of the other valid value types.

Format Definition

This property is defined by the following notation:

 x-prop = x-name *(";" icalparameter) ":" value CRLF


The following might be the ABC vendor's extension for an audio-clip form of subject property:


This document was automatically converted to XHTML using an RFC to HTML converter with the original text document at the Internet Engineering Task Force web site at .  The original text document should be referred to if there are any errors or discrepancies found in this document.

Need to test your iCalendar feeds? The iCalendar Validator provides developers and testers a method to validate their iCalendar feeds, which can take data from either a URL, file or text snippet and compare it against the RFC 5545 specification.  We believe we have one of the best iCalendar validation tools available on the internet. More information about the validator can be found here.