smooth wire to black, ribbed to white??
Smooth = hot (live) - goes to NARROW pin on polarized plug.
Ribbed = neutral - goes to WIDE pin on polarized plug.
What sort of fixture are you hanging that you're using zip cord? Is it one of those chandeliers or swag lamps? Those are the only two types of luminaires that use zip cord (running alongside a chain) to connect to the mains.
All of the flush-mount fixtures I've seen use separate black and white high-temperature "pigtail" wires for connections to the conductors in the box, NEVER zip cord. Zip cord is not rated for that sort of application and will degrade from the heat put out by the bulbs!!
Be careful and consult a licenced electrician if you are in over your head!!!
402.8 Grounded Conductor Identification.
One conductor of fixture wires that is intended to be used as a grounded conductor shall be identified by means of stripes or by the means described in 400.22(A) through (E).
400.22 Grounded-Conductor Identification.
One conductor of flexible cords that is intended to be used as a grounded circuit conductor shall have a continuous marker that readily distinguishes it from the other conductor or conductors.
The identification shall consist of one of the methods indicated in 400.22(A) through (F).