A Tweet that contains another user’s username preceded by the @ sign (e.g. @yourusername) will appear in both the public Twitter time line and the @yourusername account’s own Twitter feed.
Twitter calls this category of Tweet an @Mention.
Tweets that begin with an @Mention (e.g. ‘@username hello!’) will appear in your public Timeline but only feature in the Timeline of your followers who follow both your account and the account of the user you mention
However, Tweets that do not begin with an @Mention (e.g. ‘hello @username!’) are visible to everyone
Therefore if it is important to you that the @Mention should be placed at the beginning of a Tweet and you want that Tweet to appear in all of your followers’ timelines, begin your tweet with an unobtrusive character. A period is used most often for these purposes (e.g. ‘.hello @username’)