The restrictions on the Prussian army after 1806 limited their infantry to 12 regiments, each of 3 battalions. These 12 regiments were the cream of the crop. The Reserve regiments were formed from cadre of the line regiments and recruits who were cycled through the line regiments, a way of building up the strength of trained infantry. Later on, the first line regiment was designated guards and the other 11 regiments each moved down the list 1 place. Reserve regiments that were then designated line were regiments 12 on up. Reserve regiments were clothed in a variety of uniforms, some from British stocks, some French uniforms and some Russian, from German troops in Russian service who shifted to Prussian service.
Not allowed by treaty to form divisions, the Prussians instead formed brigades of 2 or 3 regiments that were divisions in all but name.
Infantry regiments had 3 battalions, 1 of light infantry (fusiliers) and 2 of line infantry (musketeers). There were usually a company or two of rifle-armed jagers attached.
Battalions had 4 compnaies each.