<% ... %> Ruby code; inline with output.
<%= ... %> Ruby expression; replace with result.
<%# ... %> Comment; ignored; useful in testing.
% A line of Ruby code; treated as <% .. %>; set with trim_mode argument in ERB.new.
%% Replace with % if it is the first thing on a line and % processing is used.
<%% ... %%> Replace with <% or %> , respectively.