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<?php

function twentytwelve_widgets_init() {
	register_sidebar( array(
		'name'          => __( 'Main Sidebar', 'twentytwelve' ),
		'id'            => 'sidebar-1',
		'description'   => __( 'Appears on posts and pages except the optional Front Page template, which has its own widgets', 'twentytwelve' ),
		'before_widget' => '<aside id="%1$s" class="widget %2$s">',
		'after_widget'  => '</aside>',
		'before_title'  => '<h3 class="widget-title">',
		'after_title'   => '</h3>',
	) );
}
add_action( 'widgets_init', 'twentytwelve_widgets_init' );

<?php

<?php if ( is_active_sidebar( 'sidebar-1' ) ) : ?>
	<div id="secondary" class="widget-area" role="complementary">
	<?php dynamic_sidebar( 'sidebar-1' ); ?>
	</div><!-- #secondary -->
<?php endif; ?>

<?php

<?php if ( is_active_sidebar( 'extraheaderwidget' ) ) : ?>
	<?php dynamic_sidebar( 'extraheaderwidget' ); ?>
<?php endif; ?>

<?php

function twentytwelvechild_widgets_extra() {
	register_sidebar( array(
		'name'          => __( 'HeaderWidget', 'twentytwelve' ),
		'id'            => 'extraheaderwidget',
		'description'   => __( 'Appears in the header', 'twentytwelve' ),
		'before_widget' => '<aside id="%1$s">',
		'after_widget'  => '</aside>',
		'before_title'  => '<h3>',
		'after_title'   => '</h3>',
	) );
}
add_action( 'widgets_init', 'twentytwelvechild_widgets_extra' );

<?php

function twentytwelve_widgets_extra() {
	register_sidebar( array(
		'name'          => __( 'HeaderWidget', 'twentytwelve' ),
		'id'            => 'extraheaderwidget',
		'description'   => __( 'Appears in the header', 'twentytwelve' ),
		'before_widget' => '<div id="%1$s" class="mynewheaderclass">',
		'after_widget'  => '</div;>',
		'before_title'  => '<h2>',
		'after_title'   => '</h2>',
	) );
}
add_action( 'widgets_init', 'twentytwelve_widgets_extra' );