gistfile1.aw

<?php
function additional_post_classes( $classes ) {
	global $wp_query;

	if( $wp_query->found_posts < 1 ) {
		return $classes;
	}

	if( $wp_query->current_post == 0 ) {
		$classes[] = 'post-first';//ループ内の最初の投稿にpost-firstというclass名を与える
	}

	if( $wp_query->current_post % 2 ) {
		$classes[] = 'post-even';//ループ内の偶数番目の投稿にpost-evenというclass名を与える
	} else {
		$classes[] = 'post-odd';//ループ内の奇数番目の投稿にpost-evenというclass名を与える
	}

	if( $wp_query->current_post == ( $wp_query->post_count - 1 ) ) {
		$classes[] = 'post-last';//ループ内の最後の投稿にpost-evenというclass名を与える
	}

	return $classes;
}
add_filter( 'post_class', 'additional_post_classes' );
?>