diff --git a/Gruntfile.coffee b/Gruntfile.coffee
index afee3e5..796f4cc 100644
--- a/Gruntfile.coffee
+++ b/Gruntfile.coffee
@@ -49,6 +49,7 @@ module.exports = (grunt) ->
 			'scripts/services/reader/markdown.js'
 			'scripts/services/reader/json.js'
 			'scripts/services/transformer.js'
+			'scripts/services/filter.js'
 			'scripts/controllers.js'
 		]
 		styles: [
@@ -65,6 +66,7 @@ module.exports = (grunt) ->
 			'tests/services/reader/markdown.js'
 			'tests/services/reader/json.js'
 			'tests/services/transformer.js'
+			'tests/services/filter.js'
 		]
 		test_styles: [
 			'bower_components/qunit/qunit/qunit.css'
diff --git a/source/scripts/services/filter.js.coffee b/source/scripts/services/filter.js.coffee
new file mode 100644
index 0000000..21567a2
--- /dev/null
+++ b/source/scripts/services/filter.js.coffee
@@ -0,0 +1,46 @@
+angular.module("sideBySide").service "filter", ['poems', (poems) ->
+	headers = []
+
+	@update = () ->
+		extractHeaders(poems.all)
+
+	@getFilter = () ->
+		for header in headers
+			activeHeaderValues = _.uniq(poems.getActive().map((item) ->
+				item.meta[header]
+			))
+			if (activeHeaderValues.length > 1)
+				continue # header value differs
+
+			inactiveMatching = poems.getInactive().filter((item) ->
+				item.meta[header] == activeHeaderValues[0]
+			)
+			if (inactiveMatching.length > 0)
+				continue # inactive poems have same header value
+
+			ret = {}
+			ret[header] = activeHeaderValues
+			return ret
+
+	extractHeaders = (all) ->
+		fieldLengths = all
+			.map (poem) ->
+				poem.meta
+			.reduce((fields, meta) ->
+				for key, val of meta
+					if typeof val == 'string'
+						if key not of fields
+							fields[key] = 0
+						fields[key] += val.length
+				fields
+			, {})
+
+		headers = _
+			.sortBy((
+				{ 'key': k, 'len': l } for k,l of fieldLengths
+			), 'len')
+			.map (item) ->
+				item.key
+
+	@
+]
diff --git a/source/tests/services/filter.js.coffee b/source/tests/services/filter.js.coffee
new file mode 100644
index 0000000..b403b5a
--- /dev/null
+++ b/source/tests/services/filter.js.coffee
@@ -0,0 +1,65 @@
+filter = {}
+poems = {}
+poemList = {}
+
+module "filter", {
+	setup: () ->
+		poemList.poe = { meta: {
+			Active: false
+			Author: 'Edgar Allan Poe'
+			Title: 'The Raven'
+			Year: '1845'
+			Language: 'English'
+			Code: 'poe'
+		}}
+		poemList.vrch = { meta: {
+			Active: false
+			Author: 'Jaroslav Vrchlický'
+			Title: 'Havran'
+			Year: '1845'
+			Language: 'Czech'
+			Code: 'vrch'
+		}}
+		poemList.muz = { meta: {
+			Active: false
+			Author: 'Augustin Eugen Mužík'
+			Title: 'Havran'
+			Year: '1885'
+			Language: 'Czech'
+			Code: 'muz'
+		}}
+		poemList.dolu = { meta: {
+			Active: false
+			Author: 'Karel Dostál Lutinov'
+			Title: 'Havran'
+			Year: '1918'
+			Language: 'Czech'
+			Code: 'dolu'
+		}}
+
+		angular.module('sideBySide').service('poems', () ->
+			@all = [
+				poemList.poe
+				poemList.vrch
+				poemList.muz
+				poemList.dolu
+			]
+			@getActive = () ->
+				(poem for poem in @all when poem.meta.Active is true)
+			@getInactive = () ->
+				(poem for poem in @all when poem.meta.Active is not true)
+			@
+		)
+
+		filter = angular.injector(['ng', 'sideBySide']).get('filter')
+}
+
+test "get filter for poems that share a common property", () ->
+	poemList.dolu.meta.Active = true
+	poemList.vrch.meta.Active = true
+	poemList.muz.meta.Active = true
+	filter.update()
+
+	deepEqual filter.getFilter(), {
+		'Language': [ 'Czech' ]
+	}