We'll begin with a mitre taken from circa 1160-1220 which is one of the earliest extant examples of Opus Anglicanum. This mitre is sometimes referred to as the "St. Thomas of Canterbury mitre." As you'll see, the design of this particular mitre has a mismatched lappet (the stole like appendage that hangs from the mitre) though this is due to the fact the one seen on the left is not original to this mitre; the one on the right is. So understand, first of all, that this was not a purposeful design choice.
A common design feature of mitres from the Renaissance and medieval period is the use of a decorative horizontal band around the base of the mitre (called a 'circulus') to which is attached a similar vertical strip (called a 'titulus'). In this instance, this early medieval mitre has a decorative red silk circulus and titulus. In addition to this, the main part of the mitre is covered with a cream white silk damask upon which has been set decorative embroideries. While these have now faded and subject to the ravages of time, they would have at one time been much brighter and more colourful.
The next mitre for your consideration is from circa 1300-1350 and was made in Italy. Structurally you'll once again notice the inverted 'T' design of the circulus and titulus which, in this case, includes Byzantine-esque embroideries of various saints (we can see for example on the circulus, images of St. Peter, St. Francis of Assisi and St. John the Baptist amongst others). We can also see another common design feature of medieval and Renaissance mitres, the use of precious stones and also the presence of decorative medallions. In this particular instance we see a two circular medallions with images of St. Michael and the Blessed Virgin. Both of these are done in a clear Byzantine style -- complete with the Greek inscriptions for the Mother of God.
As mentioned, the use of precious stones to add ornamental colour and texture (as well as a sense of preciousness) was common in this period (and really was so up until the most recent centuries), as for example in the case of this Neapolitan mitre made circa 1300-1325. In this instance we can see how even the edge of the mitre has decorative ornamentation applied to it. This was a design feature that was more commonplace in later medieval mitres. Once again you'll see the same inverted 'T' design as well as the circular medallions and the entire face of this mitre has been covered with small white pearls. While images of saints on mitres if very common in the medieval period, this is a good example showcasing the fact that not all mitres felt compelled to include them.
Our last mitre from this particular period is one I really quite like, coming from circa 1390-1410. It includes similar design features as our previous examples and includes figures of the apostles, Christ and the Madonna. The colours on this one are, I think, what particularly stand out to me, as well as the quality of the embroideries.
THE 1400's
As we turn our attention to the fifteenth century, we see that the designs are fairly similar still, continuing on with the use of the titulus and circulus, precious stones, medallions and so forth. Some of them continue the theme of the use of figurative depictions of saints, but we also begin to see a greater shift toward more 'abstract' designs that rest primarily on floral and vegetal motifs. This next mitre falls in the former camp however, and is dated to circa 1400-1450.
Also coming from circa 1400-1450 we have a mitre with an ornamental edging, more precious stones and metals, and no figurative imagery.
Coming from fifteenth century Lucca -- one of the important cities of the silk textile trade -- we have this beautiful mitre made of spun silver, gilded and polychrome silks. Like so many of the previous examples however, this one does include various saints on the titulus and circulus, including King David and various Old Testament prophets, as well as images of Ss. Peter and St. Paul and others. So again, we see two trends emerging in this period of history.
This back and forth trend between designs that lean more toward the symbolic and figurative designs that were favoured in the middle ages and those which shift more toward a Renaissance and especially baroque form can be especially seen in this fifteenth century mitre from Monte Cassino that was commissioned by the Medici pope, Leo X. It's design entirely rests on the use of naturalistic motifs and precious stones and pearls.
This next mitre is from circa 1500-1524 and is said to have been commissioned and donated, once again, by Pope Leo X. I think it fair to say at this point, that this style and the copious use of small pearls was clearly a style he must have preferred by the pontiff as it is the third such mitre of this type associated with him.
Lastly, here is one more example, taken from circa 1500-1550. While no precious stones form a part of this particular design, which is solely embroidered, you will see once again how the design rests entirely on floral and other vegetal motifs.
This little survey has hopefully given you at least some insight into some of the design trends that characterize mitres over the medieval and Renaissance periods. It is a good example of how the tastes of particular times help us to understand the evolution of liturgical art and design, as well as help us to understand, by sight alone, the particular period from whence an object of liturgical art may come from.