When you knit garter, you get the ridges automatically. Each purl-bump row is a ridge. They use it because it's easier to count the ridges in garter than the rows. Each ridge is 2 rows.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry