Google Gmail now offers a new streamlined compose window. When you click the red "Compose" button in Gmail, you'll be prompted to preview the new compose feature.
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To try it out, click the blue "Try it now" button at the lower left portion of the window. Otherwise, click the "Not now" button.
Should you decide you want to revert back to the old Compose, click the down arrow in the lower right corner of the Compose window and choose "Switch back to old compose" from the menu.
For more details visit: Official Google Enterprise Blog: Introducing the new compose in Gmail
For Gmail Help on this topic, visit Gmail's new compose and reply experience