What's the difference between Vespers and "Solemn" Vespers?

I can’t figure this out. Thanks!

I was curious about this too and found this under “Vespers” on Wikipedia:
Solemn Vespers before the Second Vatican Council
On weekdays that are not major feasts Vespers features hardly any ceremonies and the celebrant wears the usual choir dress. However, on Sundays and greater feasts Vespers may be solemn. Solemn Vespers differ in that the celebrant wears the cope, he is assisted by assistants also in copes, incense is used, and two acolytes, a thurifer, and at least one master of ceremonies are needed. On ordinary Sundays only two assistants are needed while on greater feasts four or six assistants may be used. The celebrant and assistants vest in the surplice and the cope, which is of the color of the day. The celebrant sits at the sedile, in front of which is placed a lectern, covered with a cloth in the color of the day. The assistants sit on benches or stools facing the altar, or if there are two assistants, they may sit at the sedile next to the celebrant (the first assistant in the place of the deacon and the second assistant in place of the subdeacon).
The celebrant and assistants follow the acolytes into the church wearing the biretta. Upon arriving in the sanctuary the acolytes place their candles on the lowest altar step, after which they are extinguished. The celebrant and assistants kneel on the lowest step and recite the Aperi Domine silently, after which they go to their places and recite the Pater noster and Ave Maria silently. A curious practice which exists from ancient times is the intoning of the antiphons and psalms to the celebrant. The rubrics presuppose that the first assistant or cantors will intone all which the celebrant must sing by singing it to him first in a soft voice after which the celebrant sings it again aloud. The five antiphons and psalms are sung with the first assistant intoning the antiphons and the cantors intoning the psalms. During the singing of the psalms all sit. After the psalms, the acolytes relight their candles and carry them to each side of the lectern for the chapter. The assistants follow, standing facing each other in front of the lectern. The celebrant then sings the chapter, after which all return to their places. The first assistant intones the hymn to the celebrant, and all stand while the hymn is sung. The first assistant intones the Magnificat to the celebrant, who sings the first line aloud. The celebrant and the first two assistants go to altar, and the altar is then incensed as at Mass while the first two assistants hold the ends of the cope. Other altars in the church may be incensed as well. The first assistant then incenses the celebrant, after which the thurifer incenses the others as at Mass. If there are commemorations, the acolytes and assistants again go to the lectern as described above for the chapter. The choir sings the antiphons, the cantors sing the versicles, and the celebrant sings the collects. After all commemorations, the celebrant sings Dominus vobiscum, the cantor sings Benedicamus Domino, and the celebrant sings Fidelium animae… The Marian antiphon is said in the low voice. Especially in English-speaking countries, Benediction of the Blessed Sacrament often follows Solemn Vespers.

Again this is from Wikipedia and the Vespers article is very long. This is just a section.

Today, Solem Vespers would probaly include a lucinarium, that is a procession with the Paschal Candle at the begining with the presider intoning “Light and peace in Jesus Christ our Lord” and everyone responding “Thanks be to God.” The presider usually wears a cope, there are servers and candles and incense. Some even have everyone holding candles that are lit from the Paschal Candle as at the Easter Vigil while a hymn is sung. The candle in incensed and an Evening Thanksgiving is chanted. Then there is the psalmody (Psalm 141/incense psalm is often done at Solemn Vespers), along with other psalms, a reading, maybe a brief homily, the Magnificat (at this time incense may be again used to incense the candle and the assembly, altar, and ministers). It concludes with the Lord’s Prayer and a final blessing and perhaps a sign of peace (no concluding hymn is necessary)

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