Prog Rock Sub-genres: Symphonic Prog

Symphonic Prog is the sub-genre that includes the most bands in progressive rock. For many people it's almost synonymous with classic prog, because most of the classic and/or pioneer bands released music that could be included in this sub-genre. Exceptions are Jethro Tull and Pink Floyd (who still blended some symphonic elements) and King Crimson who very soon expanded their horizons to more experimental music, but they included symphonic elements in their music as well.

Yes-Heart of the sunrise (Fragile, 1971 UK)

Genesis-Can-utility and the coastliners (Foxtrot, 1972 UK)

The main characteristics of Symphonic Prog are the ones that define all progressive rock which among others are:
-mixture of elements from different genres
-complex time signatures
-lush keyboards
-explorative and intelligent lyrics, in some cases close to fantasy literature, science fiction and even political issues
-non commercial approach
-longer format of songs 

Yes-Siberian khatru (Close To The Edge, 1972 UK)

Genesis-The cinema show (Selling England By The Pound, 1973 UK)

Camel-Air born (Moonmadness, 1976 UK)

In the specific case of Symphonic Prog the main characteristic is the influence of Classical music (understood as orchestral works created from the late Gothic to Modern Classical) using normally more complex structure than other related sub-genres like Neo-Prog. That's why sometimes the borderline that divides Symphonic from Neo-Prog is so unclear because it's based mostly in a degree of complexity rather than in an evident structural difference. It is easy to find long keyboard solos reminiscent of Johan Sebastian Bach or melodic works that could have been written by Handel. As in any other genre, different Symphonic Prog bands had different approaches to Classical music, for example Yes and Genesis are mainly influenced by the Baroque and Classical periods, while Emerson, Lake & Palmer has a predilection for post Romantic and modern authors like Mussorgsky, Rimsky-Korsakov, Bartók or Ginastera, as their sound is less melodic and more aggressive.

ELP-The barbarian (Emerson, Lake & Palmer, 1970 UK)

ELP-The Hut of Baba Yaga / The curse of Baba Yaga (Pictures At An Exhibition, 1971 UK)

The peak of the genre starts in 1969 and lasts until the mid/late 70's, when the genre begins to blend more mainstream influences that took to the birth of Neo-Prog (a new approach for a new decade). Even though the creative peak of Symphonic Prog ended before the 80's, there was a second wave in the 90's coming from the Scandinavian countries (specially Sweden with Änglagård) and elsewhere (like USA with Echolyn) and many of these bands still create Symphonic Prog music until present time. 

Änglagård-Jordrök (Earth smoke) (Hybris, 1992 Sweden)

Echolyn-Suffocating the bloom (Suffocating The Bloom, 1992 USA)