The company said most images uploaded are not in the square format, anyway.
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"It turns out that nearly 1 in 5 photos or videos people post aren't in the square format, and we know that it hasn't been easy to share this type of content on Instagram: friends get cut out of group shots, the subject of your video feels cramped and you can't capture the Golden Gate Bridge from end to end," the company wrote in its blog post.
The acceptance of rectangular posts from its users comes after an expansion of the styles that advertisers can use, including photos, videos and slideshows.
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Instagram has been working on targeting their ads to its users more effectively.