#include<bits/stdc++.h>
#define ll long long
#define ull unsigned long long
#define maxn 1005
#define put() putchar('\n')
#define Tp template<typename Ty>
#define Ts template<typename Ty,typename... Ar>
using namespace std;
void read(int &x){
    int f=1;x=0;char c=getchar();
    while (c<'0'||c>'9') {if (c=='-') f=-1;c=getchar();}
    while (c>='0'&&c<='9') {x=x*10+c-'0';c=getchar();}
    x*=f;
}
namespace Debug{
	Tp void _debug(char* f,Ty t){cerr<<f<<'='<<t<<endl;}
	Ts void _debug(char* f,Ty x,Ar... y){while(*f!=',') cerr<<*f++;cerr<<'='<<x<<",";_debug(f+1,y...);}
	Tp ostream& operator<<(ostream& os,vector<Ty>& V){os<<"[";for(auto& vv:V) os<<vv<<",";os<<"]";return os;}
	#define gdb(...) _debug((char*)#__VA_ARGS__,__VA_ARGS__)
}using namespace Debug;
#define fi first
#define se second
#define mk make_pair
const int mod=1e9+7;
int power(int x,int y=mod-2) {
	int sum=1;
	while (y) {
		if (y&1) sum=sum*x%mod;
		x=x*x%mod;y>>=1;
	}
	return sum;
}
int n,m,a[maxn][maxn];
void print(void) {
	int i,j;
	for (i=1;i<=n;i++,put()) for (j=1;j<=n;j++) putchar(a[i][j]?'R':'B');
}
int calc(){
	int i,j,ans=0;
	for (i=1;i<=n;i++) for (j=1;j<n;j++) ans+=(a[i][j]!=a[i][j+1]);
	for (i=1;i<n;i++) for (j=1;j<=n;j++) ans+=(a[i][j]!=a[i+1][j]);
	return ans;
}
void solve(void) {
	int i,j;
	read(n);read(m);int K=m;
	for (i=1;i<=n;i++) for (j=1;j<=n;j++) a[i][j]=0;
	if (m==1||m==2*n*(n-1)-1) return puts("Impossible"),void();
	puts("Possible");
	int tmp1=0,tmp2=0,tmp3=0;
	if (n==3) {
		if (m==3) return a[1][2]=1,print(),void();
		else if (m==5) return a[3][3]=a[1][2]=1,print(),void();
		else if (m==6) return a[1][2]=a[2][1]=1,print(),void();
		else if (m==7) return a[3][1]=a[3][3]=a[1][2]=1,print(),void();
		else if (m==8) return a[1][1]=a[1][3]=a[3][1]=a[3][3]=1,print(),void();
		else if (m==9) return a[1][2]=a[2][1]=a[2][3]=1,print(),void();
		else if (m==10) return a[1][1]=a[1][3]=a[2][2]=a[3][1]=1,print(),void();
	}
	if (n&1) {
//		tmp1=2,tmp2=(n-2)/2*3,tmp3=((n-2)*(n-2)+1)/2;
		for (i=1;i<n&&m>7;i++) 
			for (j=1;j<=n&&m>7;j++) if ((i+j)%2==0) {
				int sum=4;
				if (i==1||i==n) sum--;
				if (j==1||j==n) sum--;
				m-=sum;
				a[i][j]=1;
			}
//		gdb(m);
		if (n==5&&m==6) {
			a[n-1][n-1]=a[n][n]=1;
			print();return ;
		}
		for (j=3;j<n&&m>4;j+=2) m-=3,a[n][j]=1;
		if (m==3) a[n][j]=1;
		else if (m==2) a[n][1]=1;
		else if (m==4) a[n][1]=a[n][n]=1;
	}
	else {
		for (i=1;i<n&&m>7;i++) 
			for (j=1;j<=n&&m>7;j++) if ((i+j)%2==0&&i+j>2) {
				int sum=4;
				if (i==1||i==n) sum--;
				if (j==1||j==n) sum--;
				m-=sum;
				a[i][j]=1;
			}
		if (n==4&&m==6) {
			a[4][4]=a[3][3]=1;
			print();return ;
		}
		for (j=2;j<n&&m>4;j+=2) m-=3,a[n][j]=1;
		if (m==3) a[n][j]=1;
		else if (m==2) a[1][1]=1;
		else if (m==4) a[1][1]=a[n][n]=1;
	}
	print();
//	printf("%d\n",calc());
	assert(calc()==K);
}
signed main(void){
	int T;
	read(T);
	while (T--) solve();
	return 0;
}