diff --git a/static/css/clif.css b/static/css/clif.css
index 3a4612f..5cbebc0 100644
--- a/static/css/clif.css
+++ b/static/css/clif.css
@@ -262,6 +262,12 @@ div.threads {
     .filters .buttons {
         margin-top: 1rem;
     }
+    
+    .filters select {
+        border: 0;
+        background: transparent;
+        text-align: right;
+    }
             
 .thread {
 }
diff --git a/templates/mailing_list/emails.html b/templates/mailing_list/emails.html
index 0214e3a..6b277ec 100644
--- a/templates/mailing_list/emails.html
+++ b/templates/mailing_list/emails.html
@@ -3,8 +3,8 @@
 {% block page_title %}Mailing list: {{ list_address }}{% endblock %}
 
 {% block content %}
-    
-    <details class="filters">
+
+    <details class="filters" open>
         <summary>Filters</summary>
         
         <form action="" method="get">
@@ -12,7 +12,14 @@
                 <div class="tag">
                     <b>{{ tag }}:</b>
                     {% for value in tags[tag]|sort %}
-                        <label><input type="checkbox" name="{{ tag }}" value="{{ value }}" {{ 'checked' if value in query_tags[tag] }} /> {{ value }}</label>
+                        {% set selected = null %}
+                        {% if '+'+value in query_tags[tag] %}{% set selected = true %}{% endif %}
+                        {% if '-'+value in query_tags[tag] %}{% set selected = false %}{% endif %}
+                        <select name="{{ tag }}">
+                            <option disabled {{ 'selected' if selected == null }}>{{ value }}</option>
+                            <option {{ 'selected' if selected == true }}>+{{ value }}</option>
+                            <option {{ 'selected' if selected == false }}>-{{ value }}</option>
+                        </select>
                     {% endfor %}
                 </div>
             {% endfor %}
diff --git a/web.py b/web.py
index d3c7cbd..4b5d942 100644
--- a/web.py
+++ b/web.py
@@ -790,7 +790,16 @@ def threads(repository):
         keep = True
         for key in query_tags.keys():
             for value in query_tags[key]:
-                if value not in thread_tags.get(key, []):
+                action, value = value[0], value[1:]
+                
+                if action not in [ '+', '-' ]:
+                    bottle.abort(400, 'Bad request: {}'.format(key))
+                
+                if action == '+' and value not in thread_tags.get(key, []):
+                    keep = False
+                    break
+                
+                if action == '-' and value in thread_tags.get(key, []):
                     keep = False
                     break