Bone_Ca_id [mol] [start_residue] [direction] [id(i)]
When activated you must specify where you wish to start in the sequence and if you wish to go forward or backward through the sequence. You can then pick a Bone atom and its coordinates get associated with the CA atom of the current residue shown in the prompt window. The identified atom is usually a skeleton atom but can in fact be any kind of atom. After the atom has been identified, an object called CA will be updated, if it exists. At the start of building a new structure, there will be no sensible coordinates for any of the atoms in the molecule but the user should still create an object called CA to see what happens when using this command. The CA object can be created with the following commands
obj ca ca ; end
but no object will be seen on the screen until you start picking atoms. The Baton_build command is an alternative (and often preferred) way of building a CA trace.