In recent times, more Ukrainian composers have started to incorporate the bandura in their orchestral works, with traditional Ukrainian folk operas such as "Natalka Poltavka" being re-scored for the bandura. Contemporary works such as "Kupalo" by Y. Stankovych and "The Sacred Dnipro" by Valery Kikta also incorporated the bandura as part of the orchestra. Western composers of Ukrainian background, such as Yuriy Oliynyk and Peter Senchuk, have also composed serious works for the bandura. Today, all the conservatories of music in Ukraine offer courses majoring in bandura performance. Bandura instruction is also offered in all music colleges and most music schools, and it is now possible to get advanced degrees specialising in bandura performance and pedagogy. The most renowned of these establishments are the Kiev and Lviv conservatories and the Kiev University of Culture, primarily because of their well-established staff. Other centers of rising prominence are the Odessa Conservatory and Kharkiv University of Culture. The back of a traditional bandura is usually carved from a solid piece of wood (either willow, poplar, cherry or maple). Since the 1960s, glued-back instruments have also become common; even more recently, banduras have begun to be constructed with fiberglass backs. The soundboard is traditionally made from a type of spruce. The wrest planks and bridge are made from hard woods such as birch. The instrument was originally a diatonic instrument and, despite the addition of chromatic strings in the 1920s, it has continued to be played as a diatonic instrument. Most contemporary concert instruments have a mechanism that allows for rapid re-tuning of the instrument into different keys. These mechanisms were first included in concert instruments in the late 1950s. Significant contributions to modern bandura construction were made by Hnat Khotkevych, Leonid Haydamaka, Peter Honcharenko, Ivan Skliar, Vasyl Herasymenko and William Vetzal. Today, there are four main types of bandura which differ in construction, holding, playing technique, repertoire, and sound. The Starosvitska bandura or traditional banduras, common from the late 18th century, is also sometimes referred to as "classical" or "old-time bandura". These instruments usually have some 12-20-23 strings, tuned diatonically (4-6 bass strings and 16-18 treble strings known as prystrunky).