In my Forest Service days I made quite a few shakes for various primative buildings we maintained around the district. We used blocks of cedar and a froe and mallet and cut straight shakes. Here's a brief description from online:

Originally, all shakes were split from blocks entirely by hand using a sharp blade called a froe, and a mallet. Some shakes are still manufactured this way, typically for special orders. This splitting technique may be done straight or at an angle, depending on the needs of the customer. Shakes which are split straight to create parallel sides, have sometimes been called "barn shakes". Tapersplit shakes are split at an angle, and are only made with hand tools.

Here's a froe (without a handle) from what looks like a tool catalog. Some of the other photos out there were of rusted out artifacts. A froe can be a high-carbon steel so that it can be sharpened or of a stainless steel (the photo looks like that) that would hold a sharp edge longer (but is harder to sharpen).
Ah ha! This is the best looking one I found an image of.

I am not going to go looking for images of mallets. But take my word for it, you need a heavy wooden object like the mallet to work the fro into the top bit of the cedar block, and once it has penetrated and cracked off a slice, you pound the protruding end or work it back and forth to pry the shake off of the block. It's kind of fun to do. The work goes fast, isn't real hard, and they're a quite durable siding or roofing material.

SRS