A blue lace - The plate was fastened upon a blue band or fillet, so tied round the mitre as to show the plate in front.
The mitre - A twisted band of linen Exodus 28:39 coiled into a cap, to which the name mitre, in its original sense, closely answers, but which, in modern usage, would rather be called a turban.

And thou shalt put it on a blue lace,.... The plate of gold:
that it may be upon the mitre; either the plate or the lace; the lace is the nearest antecedent, but it seems by what follows it should be the plate:
upon the forefront of the mitre it shall be; the plate of gold; the mitre was of linen, a wrap of linen about the head, and was like a turban on it, in the top of it; it did not come down low upon the forehead, but left that bare for this plate of gold to be put upon it. Jarchi seems to understand all this of the lace, by comparing it with the following verse, and Exodus 39:31 as if the plate was represented as in the lace, and the lace upon the plate and upon the mitre above; all which he thinks is to be reconciled by observing, that the plate had three holes, and in every hole was a blue lace, and each lace was divided into two parts, so that there were six in all, two laces at each end of the plate, and two in the middle, by which they were fastened upon the top of the mitre, by which it was kept from falling off; and of this middle lace, he thinks, the text is to be understood. The Targum of Jonathan observes, that this plate was put on a blue lace, to make atonement for the impudent.

mitre--crown-like cap for the head, not covering the entire head, but adhering closely to it, composed of fine linen. The Scripture has not described its form, but from JOSEPHUS we may gather that it was conical in shape, as he distinguishes the mitres of the common priests by saying that they were not conical--that it was encircled with swathes of blue embroidered, and that it was covered by one piece of fine linen to hide the seams.