Actually it depends the grind. A hollow ground the blade cross section is concave. A flat or straight grind the sides of the blade are straight giving the cross section a wedge shape.
Pictures are from wikipedia
View attachment 240392
Full explanation quoted from wikipedia
https://en.wikipedia.org/wiki/Straight_razor
"The combination of the types found in these two classification categories can, in theory, lead to a wide variety of blade types such as round point hollow ground, square point flat ground etc., but in practice some points are combined with a specific grind. As an example, a French point blade is usually flat ground.
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![[IMG]](proxy.php?image=https%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2Fa%2Fa7%2FBlade_hollowness_scale.png%2F220px-Blade_hollowness_scale.png&hash=bb305361e7077c0a30be14211a94269d)
Degrees of hollowness. From left: Extra hollow (singing blade), 1/1 Hollow, 1/2 Hollow, 1/4 Hollow, Concave Wedge, Linear Wedge
A hollow grind produces a thinner blade than the flat grind because it removes more material from the blade (
hollows or
thins the blade more). The hollow-ground blade flexes more easily and provides more feedback on the resistance the blade meets while cutting the hair, which is an indicator of blade sharpness.
[13][43] Hollow-ground blades are preferred by most barbers and some high-end razor manufacturers limit their production exclusively to hollow ground razors.
[19][13]"Click to expand...