Isn't this where the tartans are?
The word 'plaid' (pronounced played in Scots, or plaj in Scottish Gaelic) is a noun, not an adjective. It is a common mistake to refer to the tartan sett itself as plaid. In fact, the Gaelic word plaid simply means 'blanket' or 'wrap' which is a more accurate description of what is offered here.
IMPORTANT: If you are purchasing a kilt and decide to get a plaid, it is usually best to order both at the same time. Both will be cut from the same length of fabric and this will ensure the two garments match. If you require a plaid for an existing kilt there may be some difficulties involved. If the kilt was previously ordered from me it is very likely that I can make a matching plaid. If the kilt was made by someone else, it would help considerably to know which mill wove the fabric and what weight it is. Fabric runs are not always consistent, especially with a natural product like wool. (Dyes may appear different, etc.)