diff --git a/boot/boot.js b/boot/boot.js
index 353e5e0f6..fbac37d77 100644
--- a/boot/boot.js
+++ b/boot/boot.js
@@ -892,6 +892,19 @@ $tw.modules.applyMethods = function(moduleType,targetObject) {
 	return targetObject;
 };
 
+/*
+Return a class created from a modules. The module should export the properties to be added to those of the optional base class
+*/
+$tw.modules.createClassFromModule = function(moduleExports,baseClass) {
+	var newClass = function() {};
+	if(baseClass) {
+		newClass.prototype = new baseClass();
+		newClass.prototype.constructor = baseClass;
+	}
+	$tw.utils.extend(newClass.prototype,moduleExports);
+	return newClass;
+};
+
 /*
 Return an array of classes created from the modules of a specified type. Each module should export the properties to be added to those of the optional base class
 */
@@ -899,13 +912,7 @@ $tw.modules.createClassesFromModules = function(moduleType,subType,baseClass) {
 	var classes = Object.create(null);
 	$tw.modules.forEachModuleOfType(moduleType,function(title,moduleExports) {
 		if(!subType || moduleExports.types[subType]) {
-			var newClass = function() {};
-			if(baseClass) {
-				newClass.prototype = new baseClass();
-				newClass.prototype.constructor = baseClass;
-			}
-			$tw.utils.extend(newClass.prototype,moduleExports);
-			classes[moduleExports.name] = newClass;
+			classes[moduleExports.name] = $tw.modules.createClassFromModule(moduleExports,baseClass);
 		}
 	});
 	return classes;
diff --git a/core/modules/parsers/wikiparser/wikiparser.js b/core/modules/parsers/wikiparser/wikiparser.js
index dbeed9de2..e6f860a91 100644
--- a/core/modules/parsers/wikiparser/wikiparser.js
+++ b/core/modules/parsers/wikiparser/wikiparser.js
@@ -25,6 +25,14 @@ Attributes are stored as hashmaps of the following objects:
 /*global $tw: false */
 "use strict";
 
+/*
+type: content type of text
+text: text to be parsed
+options: see below:
+	parseAsInline: true to parse text as inline instead of block
+	wiki: reference to wiki to use
+	_canonical_uri: optional URI of content if text is missing or empty
+*/
 var WikiParser = function(type,text,options) {
 	this.wiki = options.wiki;
 	var self = this;
@@ -33,19 +41,6 @@ var WikiParser = function(type,text,options) {
 		this.loadRemoteTiddler(options._canonical_uri);
 		text = $tw.language.getRawString("LazyLoadingWarning");
 	}
-	// Initialise the classes if we don't have them already
-	if(!this.pragmaRuleClasses) {
-		WikiParser.prototype.pragmaRuleClasses = $tw.modules.createClassesFromModules("wikirule","pragma",$tw.WikiRuleBase);
-		this.setupRules(WikiParser.prototype.pragmaRuleClasses,"$:/config/WikiParserRules/Pragmas/");
-	}
-	if(!this.blockRuleClasses) {
-		WikiParser.prototype.blockRuleClasses = $tw.modules.createClassesFromModules("wikirule","block",$tw.WikiRuleBase);
-		this.setupRules(WikiParser.prototype.blockRuleClasses,"$:/config/WikiParserRules/Block/");
-	}
-	if(!this.inlineRuleClasses) {
-		WikiParser.prototype.inlineRuleClasses = $tw.modules.createClassesFromModules("wikirule","inline",$tw.WikiRuleBase);
-		this.setupRules(WikiParser.prototype.inlineRuleClasses,"$:/config/WikiParserRules/Inline/");
-	}
 	// Save the parse text
 	this.type = type || "text/vnd.tiddlywiki";
 	this.source = text || "";
@@ -54,13 +49,38 @@ var WikiParser = function(type,text,options) {
 	this.configTrimWhiteSpace = false;
 	// Set current parse position
 	this.pos = 0;
-	// Instantiate the pragma parse rules
-	this.pragmaRules = this.instantiateRules(this.pragmaRuleClasses,"pragma",0);
-	// Instantiate the parser block and inline rules
-	this.blockRules = this.instantiateRules(this.blockRuleClasses,"block",0);
-	this.inlineRules = this.instantiateRules(this.inlineRuleClasses,"inline",0);
-	// Parse any pragmas
+	// Start with empty output
 	this.tree = [];
+	// Assemble the rule classes we're going to use
+	var pragmaRuleClasses, blockRuleClasses, inlineRuleClasses;
+	if(options.rules) {
+		pragmaRuleClasses = options.rules.pragma;
+		blockRuleClasses = options.rules.block;
+		inlineRuleClasses = options.rules.inline;
+	} else {
+		// Setup the rule classes if we don't have them already
+		if(!this.pragmaRuleClasses) {
+			WikiParser.prototype.pragmaRuleClasses = $tw.modules.createClassesFromModules("wikirule","pragma",$tw.WikiRuleBase);
+			this.setupRules(WikiParser.prototype.pragmaRuleClasses,"$:/config/WikiParserRules/Pragmas/");
+		}
+		pragmaRuleClasses = this.pragmaRuleClasses;
+		if(!this.blockRuleClasses) {
+			WikiParser.prototype.blockRuleClasses = $tw.modules.createClassesFromModules("wikirule","block",$tw.WikiRuleBase);
+			this.setupRules(WikiParser.prototype.blockRuleClasses,"$:/config/WikiParserRules/Block/");
+		}
+		blockRuleClasses = this.blockRuleClasses;
+		if(!this.inlineRuleClasses) {
+			WikiParser.prototype.inlineRuleClasses = $tw.modules.createClassesFromModules("wikirule","inline",$tw.WikiRuleBase);
+			this.setupRules(WikiParser.prototype.inlineRuleClasses,"$:/config/WikiParserRules/Inline/");
+		}
+		inlineRuleClasses = this.inlineRuleClasses;
+	}
+	// Instantiate the pragma parse rules
+	this.pragmaRules = this.instantiateRules(pragmaRuleClasses,"pragma",0);
+	// Instantiate the parser block and inline rules
+	this.blockRules = this.instantiateRules(blockRuleClasses,"block",0);
+	this.inlineRules = this.instantiateRules(inlineRuleClasses,"inline",0);
+	// Parse any pragmas
 	var topBranch = this.parsePragmas();
 	// Parse the text into inline runs or blocks
 	if(options.parseAsInline) {
diff --git a/editions/dev/tiddlers/new/ParserSubclassingMechanism.tid b/editions/dev/tiddlers/new/ParserSubclassingMechanism.tid
new file mode 100644
index 000000000..c4aab295e
--- /dev/null
+++ b/editions/dev/tiddlers/new/ParserSubclassingMechanism.tid
@@ -0,0 +1,42 @@
+created: 20210203150855206
+modified: 20210203150855206
+title: ParserSubclassingMechanism
+
+!! Introduction
+
+The wikitext parser subclassing mechanism makes it possible for custom parsers to use the wiki text parsing engine, but to customise the rules that are used.
+
+!! Example
+
+Here is an example of a subclass of the checkbox widget that adds logging to the event handler:
+
+```js
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+var WikiParser = require("$:/core/modules/parsers/wikiparser/wikiparser.js")["text/vnd.tiddlywiki"],
+	HtmlParser = $tw.modules.createClassFromModule(require("$:/core/modules/parsers/wikiparser/rules/html.js"),$tw.WikiRuleBase),
+	EntityParser = $tw.modules.createClassFromModule(require("$:/core/modules/parsers/wikiparser/rules/entity.js"),$tw.WikiRuleBase);
+
+var MyCustomWikiParser = function(type,text,options) {
+	var parser = new WikiParser(type,text,$tw.utils.extend({},options,{
+		// Force the parser to parse in inline mode
+		parseAsInline: true,
+		// Specify which rules will be used
+		rules: {
+			pragma: [],
+			block: [],
+			inline: [HtmlParser,EntityParser]
+		}
+	}));
+	this.tree = parser.tree;
+	this.prototype = parser.prototype;
+};
+
+exports["text/vnd.my-custom-type"] = MyCustomWikiParser;
+
+})();
+```