p.boxed
  | Giving check is forbidden, unless it is a checkmate.

p.
  Neither player is allowed to give a check, with the exception of checkmate.
  Thus, the king is much more powerful than in orthodox chess: as long as
  he can (potentially) escape, he doesn't fear attacks.
  So the king can be used to defend pieces in an unusual way.

p.
  On the following diagram, 1.Qxa6 threatens 2.Bb6
  with a mate to follow by Qxa7.
  The black rook cannot take because it would check the white king.

figure.diagram-container
  .diagram
    | fen:1k1r1b2/rP1b2p1/pQ1pp1nq/K4p1p/P4PnP/2PN2P1/3PP1B1/R1RN2B1:
  figcaption 1.Qxd8+ is forbidden because 1...Bc8 would be possible.

h3 Disambiguation

p.
  1.Qf7# is checkmate on the left diagram, because if the king takes
  then the rook on h8 gives check but not checkmate.
  However, on the right diagram 1.Qf7+ runs into 1...Kxf7#, which is now
  a legal move because the white king is checkmated.

figure.diagram-container
  .diagram.diag12
    | fen:K5kr/8/5Q2/8/8/8/8/8:
  .diagram.diag22
    | fen:K5kr/RB6/5Q2/8/8/8/8/7b:
  figcaption 1.Qf7 mates on the left, but not on the right.

h3 Source

p
  a(href="https://www.chessvariants.com/usualeq.dir/checklss.html")
    | Checkless chess
  |  on chessvariants.com, and the 
  a(href="https://en.wikipedia.org/wiki/Checkless_chess") Wikipedia page
  | .