To answer the question posed by TheGerman, the Nicolai Brothers are listed in the 1901 city directory under "Oil dealers" alongside Standard Oil. Draw your own conclusions.

If my dad were around, he would tell you that it might be an "ice" box. In college in Virginia (pre-WWII), he had one outside his third story dorm room window. He and his roommate built it to keep their sloe gin, beer, and other necessities nice and cool. Worked well in cooler and winter months, not so well in warmer or summer months.

One: My best guess is that there are two fireplaces, one at ground level and one at the first floor. Note that one of the chimneys is not like all the others, there is one much larger chimney to the left of the gas holder at the left end of the building. I wonder what that is for??

Two: Could be, but my eyesight isn't what it used to be.

Three: This might be a wooden(?) crate to store milk bottles or food/liquids that require cooler temperatures rather than an AC unit. We are looking to the southeast so this side of the building doesn't get much sunshine in the winter.

What I would like to know is what kind of oils the Nicolai Brothers are selling??? Is that oil used for heating, oil (of various density) to lubricate small and large machines or oil meant for human consumption (cooking oil etc.)??

[Nicolai Bros. supplied the District with naphtha fuel for street lighting. This liquid hydrocarbon was a byproduct of coal gasification. - Dave]

That is a window planter box and those are most likely cows grazing in the fields.
If you wanted fresh milk in 1901 it was best to have a cow nearby.

Looking at these photos always make me feel as if I was living there at that time. It looks so familiar.

[That biscuit crate under the window is nature's refrigerator. The animal in the vacant lot at the center of the frame is indeed a horse. If you wanted fresh milk, Washington was well supplied with dairies. - Dave]

One: Were the brick tenements built so that each unit had two fireplaces (hence two chimneys apiece), or were there two residential units per long narrow section (a front and a back), with one fireplace apiece? I'm inclined to believe it's the former but I'd like to know what other Shorpy sharp-eyes think.

Two: There is something standing in the field about midway down the long line of youngish trees. It looks like a horse with its head down, grazing. Do my eyes deceive me, or is it indeed an equine unit with the munchies?

Three: Beneath the far-right upstairs window of the brick tenement to the left of the wooden apartment building is what looks like an air conditioning unit. But since those weren't invented until thirty years later (I looked it up), what could it be?

Anyone?

The church at left in the background is St. Matthew’s Chapel. A check of the 1903 Baist atlas doesn’t provide any clues (to me, anyway) about the nature of the “penthouse on stilts” just north (and east?) of it, also on M Street SE.

I recall reading years ago a book titled "Washington Goes To War" written by the very talented newsman David Brinkley. In it, he described D.C. at the start of WWII as a very Southern city with many thousands of outdoor privies still in use. Plenty here in 1901, which must have made that back alley splendiferous on a hot summer day.

Anyone have an idea what that could be? It looks like a 'house on stilts' and is probably relatively modern, as that could be metal rather than wood posts. I see a smokestack as part of the structure (and that makes me wonder what they're using for fuel, possibly natural gas?)

[There was no natural gas in D.C. back then. The gas used for fuel at the time was coal gas, a.k.a. illuminating gas or "city gas." - Dave]

Someone was going to say it!

[Whose what? - Dave]

No! What's at third. Who's at second!