<a href="https://twitter.com/share" class="twitter-share-button" data-url="https://dev.twitter.com/pages/tweet_button" data-via="your_screen_name" data-lang="en" data-related="anywhereTheJavascriptAPI" data-count="vertical">Tweet</a>
<script>!function(d,s,id){var js,fjs=d.getElementsByTagName(s)[0];if(!d.getElementById(id)){js=d.createElement(s);js.id=id;js.src="//platform.twitter.com/widgets.js";fjs.parentNode.insertBefore(js,fjs);}}(document,"script","twitter-wjs");</script>
source: Tweet Button | Twitter Developers
data-count="vertical"to the<a>code. – Alex Dec 28 '11 at 14:21