According to my book of costume, trews are intended to fit close to the body, and are cut on the cross - the bias of the fabric, so as to be flexible.

Trousers are cut on the line or the stright grain of the fabric.

The link given by Southern Breeze is to what I would call trousers, not trews, as they are cut on the line of the fabric.

Patterns for the different garments would not be interchangeable due to the different properties of fabric cut straight or biased.

Trews, originally seem to be cut with very little wearing ease and for someone used to the loose cut of modern clothing they might be a bit too close for comfort.

It is probably more economical to cut trousers on the straight of the grain than trews on the bias, so a change from trews to trousers of a similar style but tailored slightly differently would have saved a bit on the cost.