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<?php  //<~ dont add me in

add_filter( 'post_class', 'themeprefix_odd_even_classes' );

function themeprefix_odd_even_classes( $classes ) {

	global $wp_query;
	if($wp_query->current_post % 2 == 0) {//http://stackoverflow.com/questions/7959247/php-test-if-number-is-odd-or-even
		$classes[] = 'odd';
	}
	else {
		$classes[] = 'even';
	}
	return $classes;
}