diff --git a/NEWS.md b/NEWS.md
index 79ec3d92fb..747eebf6bb 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -5,6 +5,9 @@
 * The `scale_name` argument in `continuous_scale()`, `discrete_scale()` and
   `binned_scale()` is soft-deprecated (@teunbrand, #1312).
 
+* In `theme()`, some elements can be specified with `rel()` to inherit from
+  `unit`-class objects in a relative fashion (@teunbrand, #3951).
+
 * `stat_ydensity()` with incomplete groups calculates the default `width` 
   parameter more stably (@teunbrand, #5396)
 
diff --git a/R/theme-elements.R b/R/theme-elements.R
index 044df52406..ba275e398c 100644
--- a/R/theme-elements.R
+++ b/R/theme-elements.R
@@ -448,12 +448,12 @@ el_def <- function(class = NULL, inherit = NULL, description = NULL) {
   axis.text.y.left    = el_def("element_text", "axis.text.y"),
   axis.text.y.right   = el_def("element_text", "axis.text.y"),
   axis.ticks.length   = el_def("unit"),
-  axis.ticks.length.x = el_def("unit", "axis.ticks.length"),
-  axis.ticks.length.x.top = el_def("unit", "axis.ticks.length.x"),
-  axis.ticks.length.x.bottom = el_def("unit", "axis.ticks.length.x"),
-  axis.ticks.length.y  = el_def("unit", "axis.ticks.length"),
-  axis.ticks.length.y.left = el_def("unit", "axis.ticks.length.y"),
-  axis.ticks.length.y.right = el_def("unit", "axis.ticks.length.y"),
+  axis.ticks.length.x = el_def(c("unit", "rel"), "axis.ticks.length"),
+  axis.ticks.length.x.top = el_def(c("unit", "rel"), "axis.ticks.length.x"),
+  axis.ticks.length.x.bottom = el_def(c("unit", "rel"), "axis.ticks.length.x"),
+  axis.ticks.length.y  = el_def(c("unit", "rel"), "axis.ticks.length"),
+  axis.ticks.length.y.left = el_def(c("unit", "rel"), "axis.ticks.length.y"),
+  axis.ticks.length.y.right = el_def(c("unit", "rel"), "axis.ticks.length.y"),
   axis.ticks.x        = el_def("element_line", "axis.ticks"),
   axis.ticks.x.top    = el_def("element_line", "axis.ticks.x"),
   axis.ticks.x.bottom = el_def("element_line", "axis.ticks.x"),
@@ -470,11 +470,11 @@ el_def <- function(class = NULL, inherit = NULL, description = NULL) {
   legend.background   = el_def("element_rect", "rect"),
   legend.margin       = el_def("margin"),
   legend.spacing      = el_def("unit"),
-  legend.spacing.x     = el_def("unit", "legend.spacing"),
-  legend.spacing.y     = el_def("unit", "legend.spacing"),
+  legend.spacing.x     = el_def(c("unit", "rel"), "legend.spacing"),
+  legend.spacing.y     = el_def(c("unit", "rel"), "legend.spacing"),
   legend.key          = el_def("element_rect", "rect"),
-  legend.key.height   = el_def("unit", "legend.key.size"),
-  legend.key.width    = el_def("unit", "legend.key.size"),
+  legend.key.height   = el_def(c("unit", "rel"), "legend.key.size"),
+  legend.key.width    = el_def(c("unit", "rel"), "legend.key.size"),
   legend.text         = el_def("element_text", "text"),
   legend.title        = el_def("element_text", "title"),
   legend.position     = el_def(c("character", "numeric", "integer")),
@@ -489,8 +489,8 @@ el_def <- function(class = NULL, inherit = NULL, description = NULL) {
   panel.background    = el_def("element_rect", "rect"),
   panel.border        = el_def("element_rect", "rect"),
   panel.spacing       = el_def("unit"),
-  panel.spacing.x     = el_def("unit", "panel.spacing"),
-  panel.spacing.y     = el_def("unit", "panel.spacing"),
+  panel.spacing.x     = el_def(c("unit", "rel"), "panel.spacing"),
+  panel.spacing.y     = el_def(c("unit", "rel"), "panel.spacing"),
   panel.grid.major.x  = el_def("element_line", "panel.grid.major"),
   panel.grid.major.y  = el_def("element_line", "panel.grid.major"),
   panel.grid.minor.x  = el_def("element_line", "panel.grid.minor"),
diff --git a/R/theme.R b/R/theme.R
index 2bfdd5a835..33cf0244cd 100644
--- a/R/theme.R
+++ b/R/theme.R
@@ -724,6 +724,20 @@ combine_elements <- function(e1, e2) {
     return(e2)
   }
 
+  # Inheritance of rel objects
+  if (is.rel(e1)) {
+    # Both e1 and e2 are rel, give product as another rel
+    if (is.rel(e2)) {
+      return(rel(unclass(e1) * unclass(e2)))
+    }
+    # If e2 is a unit/numeric, return modified unit/numeric
+    # Note that unit objects are considered numeric
+    if (is.numeric(e2) || is.unit(e2)) {
+      return(unclass(e1) * e2)
+    }
+    return(e1)
+  }
+
   # If neither of e1 or e2 are element_* objects, return e1
   if (!inherits(e1, "element") && !inherits(e2, "element")) {
     return(e1)
diff --git a/_pkgdown.yml b/_pkgdown.yml
index 8276a5ded9..7dbedc3062 100644
--- a/_pkgdown.yml
+++ b/_pkgdown.yml
@@ -285,7 +285,7 @@ news:
 
 navbar:
   structure:
-    left: [reference, news, articles, extensions]
+    left: [intro, reference, news, articles, extensions]
   components:
     extensions:
       text: Extensions
diff --git a/tests/testthat/test-theme.R b/tests/testthat/test-theme.R
index 2ae6a2bef4..7bf37c90e1 100644
--- a/tests/testthat/test-theme.R
+++ b/tests/testthat/test-theme.R
@@ -170,6 +170,16 @@ test_that("calculating theme element inheritance works", {
   e1 <- ggplot2:::calc_element("strip.text.x", theme)
   e2 <- ggplot2:::calc_element("strip.text", theme)
   expect_identical(e1, e2)
+
+  # Check that rel units are computed appropriately
+  theme <- theme_gray() +
+    theme(axis.ticks.length = unit(1, "cm"),
+          axis.ticks.length.x = rel(0.5),
+          axis.ticks.length.x.bottom = rel(4))
+
+  expect_equal(calc_element("axis.ticks.length.y.left", theme), unit(1, "cm"))
+  expect_equal(calc_element("axis.ticks.length.x.top", theme), unit(1, "cm") * 0.5)
+  expect_equal(calc_element("axis.ticks.length.x.bottom", theme), unit(1, "cm") * 0.5 * 4)
 })
 
 test_that("complete and non-complete themes interact correctly with each other", {