Union Square

Weidner 116 looks west at the pre-quake St. Francis Hotel (caption typo "San Francis"), completed in 1904. The image dates from early 1904. St. Francis Hotel shows many additional views of the hotel.
Goldsmith 21 and
Britton & Rey 548 look southeast across Union Square from Powell/Post. Each shows the Mutual Savings Bank building (Market/Geary/Kearny, completed 1902, here just left of the tower of the Call Building) but not the Whitell building (170 Geary, under construction in 1906 as seen below), so these images must date from 1902-1905.
Britton & Rey 549, postmarked 11/02/1905, looks northwest from Union Square. The Hotel Savoy is on the northeast corner of Powell/Post, with the Hopkins Institute, Fairmont Hotel (under construction), and Stanford mansion above it atop Nob Hill.

Weidner 239 looks east across Union Square toward Stockton/Geary on earthquake day 4/18/1906, with the fire rapidly approaching. The gathered crowd might include Enrico Caruso, who was staying at the Palace Hotel when the earthquake struck. The steel framework of the Whitell building under construction (at center) is a useful point of reference in post-quake downtown photos.
Weidner 217, postmarked SF 3/12/1907, looks west across rubble-strewn Union Square after the earthquake and fire. The St. Francis was also under construction, adding a third wing, when the earthquake struck.
A.H., postmarked 8/30/1906, is miscaptioned; it looks east across Union Square toward Stockton/Post, so it does not show the rear of the St. Francis.