Google PRINCIPLES OF MOBILE SITE DESIGN: DELIGHT USERS AND DRIVE CONVERSIONS - Manual (Page 7)

1. CALLS-TO-ACTION
FRONT AND CENTER
It can be easy for mobile users to miss menu items,
so always put your key calls-to-action where you
know users will see them. Study participants had
the easiest time completing tasks on sites that
clearly displayed primary calls-to-action in the main
body of the site, with secondary tasks available
through menus or below the fold. Your mobile
calls-to-action will probably be different than on
desktop, so put yourself in your users’ shoes when
determining placement.
Example from Progressive Mobile Site.
Key Takeaway
Feature your primary calls-to-action
in your most prominent site space.
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