diff --git a/wp-content/themes/default/archive.php b/wp-content/themes/default/archive.php
index 8ec2e2b58a..9257ddddaa 100644
--- a/wp-content/themes/default/archive.php
+++ b/wp-content/themes/default/archive.php
@@ -28,7 +28,7 @@
 		</div>
 
 		<?php while (have_posts()) : the_post(); ?>
-		<div class="post">
+		<div <?php post_class() ?>>
 				<h3 id="post-<?php the_ID(); ?>"><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></h3>
 				<small><?php the_time('l, F jS, Y') ?></small>
 
diff --git a/wp-content/themes/default/search.php b/wp-content/themes/default/search.php
index 910b65ec8a..17ee99c5e1 100644
--- a/wp-content/themes/default/search.php
+++ b/wp-content/themes/default/search.php
@@ -14,7 +14,7 @@
 
 		<?php while (have_posts()) : the_post(); ?>
 
-			<div class="post">
+			<div <?php post_class() ?>>
 				<h3 id="post-<?php the_ID(); ?>"><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></h3>
 				<small><?php the_time('l, F jS, Y') ?></small>
 
diff --git a/wp-content/themes/default/single.php b/wp-content/themes/default/single.php
index b3bf7863c7..5c09870e1a 100644
--- a/wp-content/themes/default/single.php
+++ b/wp-content/themes/default/single.php
@@ -9,7 +9,7 @@
 			<div class="alignright"><?php next_post_link('%link &raquo;') ?></div>
 		</div>
 
-		<div class="post" id="post-<?php the_ID(); ?>">
+		<div <?php post_class() ?> id="post-<?php the_ID(); ?>">
 			<h2><?php the_title(); ?></h2>
 
 			<div class="entry">
diff --git a/wp-includes/post-template.php b/wp-includes/post-template.php
index 260f3f7ede..33e466dd58 100644
--- a/wp-includes/post-template.php
+++ b/wp-includes/post-template.php
@@ -173,16 +173,37 @@ function has_excerpt( $id = 0 ) {
  * @param int $post_id An optional post ID
  */
 function post_class( $class = '', $post_id = null ) {
+	$post = get_post($post_id);
 
-	$classes = 'post';
+	$classes[] = $post->post_type;
 
-	if ( is_sticky($post_id) )
-		$classes .= ' sticky';
+	if ( is_sticky($post->ID) )
+		$classes[] = 'sticky';
 
-	if ( !empty($class) )
-		$classes .= ' ' . $class;
+	// hentry for hAtom compliace
+	$classes[] = 'hentry';
 
-	$classes = apply_filters('post_class', $classes, $class, $post_id);
+	// Categories
+	foreach ( (array) get_the_category($post->ID) as $cat ) {
+		if ( empty($cat->slug ) )
+			continue;
+		$classes[] = 'category-' . $cat->slug;
+	}
+
+	// Tags
+	foreach ( (array) get_the_tags($post->ID) as $tag ) {
+		if ( empty($tag->slug ) )
+			continue;
+		$classes[] = 'tag-' . $tag->slug;
+	}
+
+	if ( !empty($class) ) {
+		$class = preg_split('#\s+#', $class);
+		$classes = array_merge($classes, $class);
+	}
+
+	// Separates classes with a single space, collates classes for post DIV
+	$classes = join(' ', apply_filters('post_class', $classes, $class, $post_id));
 
 	echo 'class="' . $classes . '"';
 }