Wilton Cake Icer tip
The 789 tip is an extra large piping tip that you can use to pipe icing over your whole cake. No need to crumb coat! It looks like a giant basketweave tip, pipe icing around the outside of your cake with the textured side down, and then smooth over the top with a spatula or scraper.
You need an extra large piping bag for this tip, at least 18", otherwise you will be refilling the bag constantly.
Lots of people use the same method to buttercream their cakes using just a large round tip or a tipless bag, but this tip is about 3 times as wide so is much faster! Plus with it being a wide and thin opening, it creates an even layer over your whole cake.