diff --git a/src/Logic/Tags/TagUtils.ts b/src/Logic/Tags/TagUtils.ts
index 55b142506..0a81a27a6 100644
--- a/src/Logic/Tags/TagUtils.ts
+++ b/src/Logic/Tags/TagUtils.ts
@@ -126,23 +126,7 @@ export class TagUtils {
                 "\n" +
                 "An assigning tag _cannot_ be used to query OpenStreetMap/Overpass.\n" +
                 "\n" +
-                "If using a key or variable which might not be defined, add a condition in the mapping to hide the option. This is\n" +
-                "because, if `some_other_key` is not defined, one might actually upload the literal text `key={some_other_key}` to OSM -\n" +
-                "which we do not want.\n" +
-                "\n" +
-                "To mitigate this, use:\n" +
-                "\n" +
-                "```json\n" +
-                "{\n" +
-                '    "mappings": [\n' +
-                "        {\n" +
-                '            "if":"key:={some_other_key}",\n' +
-                '            "then": "...",\n' +
-                '            "hideInAnswer": "some_other_key="\n' +
-                "        }\n" +
-                "    ]\n" +
-                "}\n" +
-                "```\n" +
+                "It is possible to assign a default value with `key_to_assing:={some_other_key??fallback_value}`. If `some_other_key` is not present, then the literal value `fallback_value` will be used instead." +
                 "\n" +
                 "One can use `key!:=prefix-{other_key}-postfix` as well, to match if `key` is _not_ the same\n" +
                 "as `prefix-{other_key}-postfix` (with `other_key` substituted by the value)",
@@ -263,8 +247,8 @@ export class TagUtils {
         return tags
     }
 
-    static SplitKeys(tagsFilters: UploadableTag[]): Record<string, string[]> {
-        return this.SplitKeysRegex(tagsFilters, false)
+    static SplitKeys(tagsFilters: UploadableTag[], currentProperties: Tags): Record<string, string[]> {
+        return this.SplitKeysRegex(tagsFilters, false, currentProperties)
     }
 
     /***
@@ -272,24 +256,31 @@ export class TagUtils {
      *
      * TagUtils.SplitKeysRegex([new Tag("isced:level", "bachelor; master")], true) // => {"isced:level": ["bachelor","master"]}
      */
-    static SplitKeysRegex(tagsFilters: UploadableTag[], allowRegex: false): Record<string, string[]>
+    static SplitKeysRegex(tagsFilters: ReadonlyArray<UploadableTag>, allowRegex: false,
+                          currentProperties: Tags): Record<string, string[]>
     static SplitKeysRegex(
-        tagsFilters: UploadableTag[],
-        allowRegex: boolean
+        tagsFilters: ReadonlyArray<UploadableTag>,
+        allowRegex: boolean,
+        currentProperties: Tags
     ): Record<string, (string | RegexTag)[]>
     static SplitKeysRegex(
-        tagsFilters: UploadableTag[],
-        allowRegex: boolean
+        tagsFiltersIn: ReadonlyArray<UploadableTag>,
+        allowRegex: boolean,
+        currentProperties: Tags
     ): Record<string, (string | RegexTag)[]> {
         const keyValues: Record<string, (string | RegexTag)[]> = {}
-        tagsFilters = [...tagsFilters] // copy all, use as queue
+        const tagsFilters = [...tagsFiltersIn] // copy all, use as queue
         while (tagsFilters.length > 0) {
-            const tagsFilter = tagsFilters.shift()
+            let tagsFilter = tagsFilters.shift()
 
             if (tagsFilter === undefined) {
                 continue
             }
 
+            if (tagsFilter instanceof SubstitutingTag) {
+                tagsFilter = (<SubstitutingTag>tagsFilter).asTag(currentProperties)
+            }
+
             if (tagsFilter instanceof And) {
                 tagsFilters.push(...(<UploadableTag[]>tagsFilter.and))
                 continue
@@ -360,12 +351,12 @@ export class TagUtils {
      * TagUtils.FlattenMultiAnswer(([new Tag("x","y"), new Tag("a","b")])) // => [new Tag("x","y"), new Tag("a","b")]
      * TagUtils.FlattenMultiAnswer(([new Tag("x","")])) // => [new Tag("x","")]
      */
-    static FlattenMultiAnswer(tagsFilters: UploadableTag[]): UploadableTag[] {
+    static FlattenMultiAnswer(tagsFilters: UploadableTag[], currentProperties: Tags): UploadableTag[] {
         if (tagsFilters === undefined) {
             return []
         }
 
-        const keyValues = TagUtils.SplitKeys(tagsFilters)
+        const keyValues = TagUtils.SplitKeys(tagsFilters, currentProperties)
         const and: UploadableTag[] = []
         for (const key in keyValues) {
             const values = Lists.dedup(keyValues[key]).filter((v) => v !== "")
@@ -387,7 +378,7 @@ export class TagUtils {
      * TagUtils.MatchesMultiAnswer(new Tag("isced:level","master"), {"isced:level":"bachelor; master"}) // => true
      */
     static MatchesMultiAnswer(tag: UploadableTag, properties: Tags): boolean {
-        const splitted = TagUtils.SplitKeysRegex([tag], true)
+        const splitted = TagUtils.SplitKeysRegex([tag], true, properties)
         for (const splitKey in splitted) {
             const neededValues = splitted[splitKey]
             if (properties[splitKey] === undefined) {
diff --git a/src/Models/ThemeConfig/TagRenderingConfig.ts b/src/Models/ThemeConfig/TagRenderingConfig.ts
index 29b342ed2..e188cdde7 100644
--- a/src/Models/ThemeConfig/TagRenderingConfig.ts
+++ b/src/Models/ThemeConfig/TagRenderingConfig.ts
@@ -897,7 +897,7 @@ export default class TagRenderingConfig {
                     ])
                 )
             }
-            const and = TagUtils.FlattenMultiAnswer([...selectedMappings, ...unselectedMappings])
+            const and = TagUtils.FlattenMultiAnswer([...selectedMappings, ...unselectedMappings], currentProperties)
             if (and.length === 0) {
                 return undefined
             }
diff --git a/src/Utils.ts b/src/Utils.ts
index 123818e2c..7ddf45d7c 100644
--- a/src/Utils.ts
+++ b/src/Utils.ts
@@ -300,6 +300,10 @@ In the case that MapComplete is pointed to the testing grounds, the edit will be
      * Utils.SubstituteKeys("abc{def}ghi", {def: '{XYZ}'}) // => "abc{XYZ}ghi"
      * Utils.SubstituteKeys("abc\n\n{def}ghi", {def: '{XYZ}'}) // => "abc\n\n{XYZ}ghi"
      *
+     * // Should support a default value
+     * Utils.SubstituteKeys("abc{def??XXX}ghi", {def: 'XYZ'}) // => "abcXYZghi"
+     * Utils.SubstituteKeys("abc{def??XXX}ghi", {randomKey: 'XYZ'}) // => "abcXXXghi"
+     *
      * @param txt
      * @param tags
      * @param useLang
@@ -322,8 +326,13 @@ In the case that MapComplete is pointed to the testing grounds, the edit will be
         }
         let result = ""
         while (match) {
-            const [_, normal, key, leftover] = match
-            let v = tags?.[key]
+            const [_, normal, keyFallback, leftover] = match
+            let key = keyFallback
+            let fallback = ""
+            if (keyFallback.indexOf("??") >= 0) {
+                [key, fallback] = keyFallback.split("??")
+            }
+            let v = tags?.[key] ?? fallback
             if (v !== undefined && v !== null) {
                 if (v["toISOString"] != undefined) {
                     // This is a date, probably the timestamp of the object