show the principles in accordance with which this invention effects pick and impression resistant characteristics in particular fig3 shows a driver pin 61 in contact with a tumbler pin 71 with which it is urged into contact by compressed helical spring 51 the key shank 12 rests within key slot 13 and presses the lower proximate portion of tumbler pin 71 upward to position the combined pin pair assembly with the parting line between driver 61 and tumbler 71 at the shear line between plug 21 and cylinder 20 it will be appreciated that when all pin assemblies are similarly acted upon by an appropriate key 11 there will be no impedance against rotation of plug 21 and the lock can be opened this presupposes that the side locking bar 16 is not forced outward into engagement with the slot 37 in the side of the cylinder the cross sectional view of fig4 is taken along line 4 4 of fig3 and shows the relative positions of the pin freeze bar 16 biasing springs 17 18 and the tumbler bores 41 45 within plug 21 the enlarged view of fig5 shows that when the cylinder is in its typical locked position the drive pin 61 projects into the plug bore where it.