What is a pure checkmate? Recently I heard something about pure checkmate. This is a special kind of checkmate where each of the squares around the king has to be covered by just one single piece. One simple example of this would be checkmate using two pawns and the king:
[FEN &quot;3k4/2PP4/3K4/8/8/8/8/8 w - - 0 1&quot;]

Unfortunately, I couldn't find any more detailed information about this topic, which is the main reason for this question.
To make the question slightly more specific, I'd like to know what are the exact circumstances for a mate to be considered pure?
Is it allowed to have opponent's pieces involved in the check mate? For example like this:
[FEN &quot;1r1q1r2/4R1p1/2p2pkp/p4Q1N/n2PP3/P5N1/5PPP/4R1K1 w - - 0 1&quot;]
According to Wikipedia:

&quot;A pure mate is a checkmating position in chess in which the mated king
and all vacant squares in its field are attacked only once, and
squares in the king's field occupied by friendly units are not also
attacked by the mating side (unless such a unit is necessarily pinned
to the king to avoid it interposing to block the check or capturing of
mating unit.&quot;

Essentially, a pure mate is a position in which each square around the opposing king is attacked only once by the mating side, so your first example is indeed a pure mate. However, your second example is not. While kings can be blocked off by their own units, they cannot be attacked unless they are pinned, which they aren't.
To summarize, the Black king can be surrounded by its pawns and pieces so long as they are not attacked, and if they are, they must be pinned. Furthermore, the pin must prevent the pinned piece from interfering with the mate.
As far as I can tell, multiple pieces may be on the board so long as the stipulations of a pure mate are met. The enemy pieces surrounding the king’s squares can be attacked, so long so as the mating piece is actually capable of mating, i.e. it cannot be captured or blocked.
Wikipedia gives an example from the final position from the famous Evergreen Game (link the chessgames.com).
 [Title &quot;”The Evergreen Game,” Adolf Anderssen-Jean Dufresne. Berlin GER, 1852&quot;]
[FEN &quot;1r3kr1/pbpBBp1p/1b3P2/8/8/2P2q2/P4PPP/3R2K1 b - - 0 1&quot;]

In this position, each of the Black king's available squares that it could move to are attacked exactly once by the White pieces, and the black blocking pieces are not attacked by white pieces at all. Although the White pawn that blocks off the g7 square for the black king is attacked, this is a real pure mate from high-level play nonetheless.
If you wish to see a constructed position for a pure mate, @DM has a great position in his answer to a related question on this site.